diff --git a/src/Error/Error.php b/src/Error/Error.php
index e31ccf1..56241d9 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -74,21 +74,36 @@ class Error extends \Exception implements \JsonSerializable
      */
     public static function createLocatedError($error, $nodes = null, $path = null)
     {
-        if ($error instanceof self) {
+        if ($error instanceof self && $error->path) {
             return $error;
         }
 
-        if ($error instanceof \Exception) {
+        $source = $positions = $originalError = null;
+
+        if ($error instanceof self) {
             $message = $error->getMessage();
-            $previous = $error;
+            $originalError = $error;
+            $nodes = $error->nodes ?: $nodes;
+            $source = $error->source;
+            $positions = $error->positions;
+        } else if ($error instanceof \Exception) {
+            $message = $error->getMessage();
+            $originalError = $error;
         } else {
             $message = (string) $error;
-            $previous = null;
         }
 
-        return new static($message, $nodes, null, null, $path, $previous);
+        return new static(
+            $message ?: 'An unknown error occurred.',
+            $nodes,
+            $source,
+            $positions,
+            $path,
+            $originalError
+        );
     }
 
+
     /**
      * @param Error $error
      * @return array
diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index f81212a..e7de7a0 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -338,7 +338,7 @@ class Executor
             : null;
 
         if ($skipAST) {
-            $argValues = Values::getArgumentValues($skipDirective->args, $skipAST->arguments, $exeContext->variableValues);
+            $argValues = Values::getArgumentValues($skipDirective, $skipAST, $exeContext->variableValues);
             if (isset($argValues['if']) && $argValues['if'] === true) {
                 return false;
             }
@@ -352,7 +352,7 @@ class Executor
             : null;
 
         if ($includeAST) {
-            $argValues = Values::getArgumentValues($includeDirective->args, $includeAST->arguments, $exeContext->variableValues);
+            $argValues = Values::getArgumentValues($includeDirective, $includeAST, $exeContext->variableValues);
             if (isset($argValues['if']) && $argValues['if'] === false) {
                 return false;
             }
@@ -410,14 +410,6 @@ class Executor
 
         $returnType = $fieldDef->getType();
 
-        // Build hash of arguments from the field.arguments AST, using the
-        // variables scope to fulfill any variable references.
-        $args = Values::getArgumentValues(
-            $fieldDef->args,
-            $fieldAST->arguments,
-            $exeContext->variableValues
-        );
-
         // The resolve function's optional third argument is a collection of
         // information about the current execution state.
         $info = new ResolveInfo([
@@ -449,7 +441,15 @@ class Executor
 
         // Get the resolve function, regardless of if its result is normal
         // or abrupt (error).
-        $result = self::resolveOrError($resolveFn, $source, $args, $context, $info);
+        $result = self::resolveOrError(
+            $exeContext,
+            $fieldDef,
+            $fieldAST,
+            $resolveFn,
+            $source,
+            $context,
+            $info
+        );
 
         $result = self::completeValueCatchingError(
             $exeContext,
@@ -463,11 +463,30 @@ class Executor
         return $result;
     }
 
-    // Isolates the "ReturnOrAbrupt" behavior to not de-opt the `resolveField`
-    // function. Returns the result of resolveFn or the abrupt-return Error object.
-    private static function resolveOrError($resolveFn, $source, $args, $context, $info)
+    /**
+     * Isolates the "ReturnOrAbrupt" behavior to not de-opt the `resolveField`
+     * function. Returns the result of resolveFn or the abrupt-return Error object.
+     *
+     * @param ExecutionContext $exeContext
+     * @param FieldDefinition $fieldDef
+     * @param Field $fieldAST
+     * @param callable $resolveFn
+     * @param mixed $source
+     * @param mixed $context
+     * @param ResolveInfo $info
+     * @return \Exception|mixed
+     */
+    private static function resolveOrError($exeContext, $fieldDef, $fieldAST, $resolveFn, $source, $context, $info)
     {
         try {
+            // Build hash of arguments from the field.arguments AST, using the
+            // variables scope to fulfill any variable references.
+            $args = Values::getArgumentValues(
+                $fieldDef,
+                $fieldAST,
+                $exeContext->variableValues
+            );
+
             return call_user_func($resolveFn, $source, $args, $context, $info);
         } catch (\Exception $error) {
             return $error;
@@ -783,8 +802,9 @@ class Executor
         $i = 0;
         $tmp = [];
         foreach ($result as $item) {
-            $path[] = $i++;
-            $tmp[] = self::completeValueCatchingError($exeContext, $itemType, $fieldASTs, $info, $path, $item);
+            $fieldPath = $path;
+            $fieldPath[] = $i++;
+            $tmp[] = self::completeValueCatchingError($exeContext, $itemType, $fieldASTs, $info, $fieldPath, $item);
         }
         return $tmp;
     }
diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index 18bba17..42f4e3d 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -5,11 +5,16 @@ namespace GraphQL\Executor;
 use GraphQL\Error\Error;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\Argument;
+use GraphQL\Language\AST\Field;
 use GraphQL\Language\AST\NullValue;
+use GraphQL\Language\AST\Value;
+use GraphQL\Language\AST\Variable;
 use GraphQL\Language\AST\VariableDefinition;
 use GraphQL\Language\Printer;
 use GraphQL\Schema;
+use GraphQL\Type\Definition\Directive;
 use GraphQL\Type\Definition\FieldArgument;
+use GraphQL\Type\Definition\FieldDefinition;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InputType;
 use GraphQL\Type\Definition\LeafType;
@@ -17,6 +22,7 @@ use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Utils;
+use GraphQL\Validator\DocumentValidator;
 
 class Values
 {
@@ -33,50 +39,126 @@ class Values
      */
     public static function getVariableValues(Schema $schema, $definitionASTs, array $inputs)
     {
-        $values = [];
-        foreach ($definitionASTs as $defAST) {
-            $varName = $defAST->variable->name->value;
-            $values[$varName] = self::getvariableValue($schema, $defAST, isset($inputs[$varName]) ? $inputs[$varName] : null);
+        $coercedValues = [];
+        foreach ($definitionASTs as $definitionAST) {
+            $varName = $definitionAST->variable->name->value;
+            $varType = Utils\TypeInfo::typeFromAST($schema, $definitionAST->type);
+
+            if (!Type::isInputType($varType)) {
+                throw new Error(
+                    'Variable "$'.$varName.'" expected value of type ' .
+                    '"' . Printer::doPrint($definitionAST->type) . '" which cannot be used as an input type.',
+                    [$definitionAST->type]
+                );
+            }
+
+            if (!array_key_exists($varName, $inputs)) {
+                $defaultValue = $definitionAST->defaultValue;
+                if ($defaultValue) {
+                    $coercedValues[$varName] = Utils\AST::valueFromAST($defaultValue, $varType);
+                }
+                if ($varType instanceof NonNull) {
+                    throw new Error(
+                        'Variable "$'.$varName .'" of required type ' .
+                        '"'. Utils::printSafe($varType) . '" was not provided.',
+                        [$definitionAST]
+                    );
+                }
+            } else {
+                $value = $inputs[$varName];
+                $errors = self::isValidPHPValue($value, $varType);
+                if (!empty($errors)) {
+                    $message = "\n" . implode("\n", $errors);
+                    throw new Error(
+                        'Variable "$' . $varName . '" got invalid value ' .
+                        json_encode($value) . '.' . $message,
+                        [$definitionAST]
+                    );
+                }
+
+                $coercedValue = self::coerceValue($varType, $value);
+                Utils::invariant($coercedValue !== Utils::undefined(), 'Should have reported error.');
+                $coercedValues[$varName] = $coercedValue;
+            }
         }
-        return $values;
+        return $coercedValues;
     }
 
     /**
      * Prepares an object map of argument values given a list of argument
      * definitions and list of argument AST nodes.
      *
-     * @param FieldArgument[] $argDefs
-     * @param Argument[] $argASTs
+     * @param FieldDefinition|Directive $def
+     * @param Field|\GraphQL\Language\AST\Directive $node
      * @param $variableValues
      * @return array
+     * @throws Error
      */
-    public static function getArgumentValues($argDefs, $argASTs, $variableValues)
+    public static function getArgumentValues($def, $node, $variableValues)
     {
-        if (!$argDefs) {
+        $argDefs = $def->args;
+        $argASTs = $node->arguments;
+
+        if (!$argDefs || null === $argASTs) {
             return [];
         }
-        $argASTMap = $argASTs ? Utils::keyMap($argASTs, function ($arg) {
+
+        $coercedValues = [];
+        $undefined = Utils::undefined();
+
+        /** @var Argument[] $argASTMap */
+        $argASTMap = $argASTs ? Utils::keyMap($argASTs, function (Argument $arg) {
             return $arg->name->value;
         }) : [];
-        $result = [];
+
         foreach ($argDefs as $argDef) {
             $name = $argDef->name;
-            $valueAST = isset($argASTMap[$name]) ? $argASTMap[$name]->value : null;
-            $value = Utils\AST::valueFromAST($valueAST, $argDef->getType(), $variableValues);
+            $argType = $argDef->getType();
+            $argumentAST = isset($argASTMap[$name]) ? $argASTMap[$name] : null;
 
-            if (null === $value && null === $argDef->defaultValue && !$argDef->defaultValueExists()) {
-                continue;
-            }
+            if (!$argumentAST) {
+                if ($argDef->defaultValueExists()) {
+                    $coercedValues[$name] = $argDef->defaultValue;
+                } else if ($argType instanceof NonNull) {
+                    throw new Error(
+                        'Argument "' . $name . '" of required type ' .
+                        '"' . Utils::printSafe($argType) . '" was not provided.',
+                        [$node]
+                    );
+                }
+            } else if ($argumentAST->value instanceof Variable) {
+                $variableName = $argumentAST->value->name->value;
 
-            if (null === $value) {
-                $value = $argDef->defaultValue;
+                if ($variableValues && array_key_exists($variableName, $variableValues)) {
+                    // Note: this does not check that this variable value is correct.
+                    // This assumes that this query has been validated and the variable
+                    // usage here is of the correct type.
+                    $coercedValues[$name] = $variableValues[$variableName];
+                } else if ($argDef->defaultValueExists()) {
+                    $coercedValues[$name] = $argDef->defaultValue;
+                } else if ($argType instanceof NonNull) {
+                    throw new Error(
+                        'Argument "' . $name . '" of required type "' . Utils::printSafe($argType) . '" was ' .
+                        'provided the variable "$' . $variableName . '" which was not provided ' .
+                        'a runtime value.',
+                        [ $argumentAST->value ]
+                    );
+                }
+            } else {
+                $valueAST = $argumentAST->value;
+                $coercedValue = Utils\AST::valueFromAST($valueAST, $argType, $variableValues);
+                if ($coercedValue === $undefined) {
+                    $errors = DocumentValidator::isValidLiteralValue($argType, $valueAST);
+                    $message = !empty($errors) ? ("\n" . implode("\n", $errors)) : '';
+                    throw new Error(
+                        'Argument "' . $name . '" got invalid value ' . Printer::doPrint($valueAST) . '.' . $message,
+                        [ $argumentAST->value ]
+                    );
+                }
+                $coercedValues[$name] = $coercedValue;
             }
-            if (NullValue::getNullValue() === $value) {
-                $value = null;
-            }
-            $result[$name] = $value;
         }
-        return $result;
+        return $coercedValues;
     }
 
     /**
@@ -92,56 +174,6 @@ class Values
         return Utils\AST::valueFromAST($valueAST, $type, $variables);
     }
 
-    /**
-     * Given a variable definition, and any value of input, return a value which
-     * adheres to the variable definition, or throw an error.
-     */
-    private static function getVariableValue(Schema $schema, VariableDefinition $definitionAST, $input)
-    {
-        $type = Utils\TypeInfo::typeFromAST($schema, $definitionAST->type);
-        $variable = $definitionAST->variable;
-
-        if (!$type || !Type::isInputType($type)) {
-            $printed = Printer::doPrint($definitionAST->type);
-            throw new Error(
-                "Variable \"\${$variable->name->value}\" expected value of type " .
-                "\"$printed\" which cannot be used as an input type.",
-                [ $definitionAST ]
-            );
-        }
-
-        $inputType = $type;
-        $errors = self::isValidPHPValue($input, $inputType);
-
-        if (empty($errors)) {
-            if (null === $input) {
-                $defaultValue = $definitionAST->defaultValue;
-                if ($defaultValue) {
-                    $value = Utils\AST::valueFromAST($defaultValue, $inputType);
-                    return $value === NullValue::getNullValue() ? null : $value;
-                }
-            }
-            return self::coerceValue($inputType, $input);
-        }
-
-        if (null === $input) {
-            $printed = Printer::doPrint($definitionAST->type);
-
-            throw new Error(
-                "Variable \"\${$variable->name->value}\" of required type " .
-                "\"$printed\" was not provided.",
-                [ $definitionAST ]
-            );
-        }
-        $message = $errors ? "\n" . implode("\n", $errors) : '';
-        $val = json_encode($input);
-        throw new Error(
-            "Variable \"\${$variable->name->value}\" got invalid value ".
-            "{$val}.{$message}",
-            [ $definitionAST ]
-        );
-    }
-
     /**
      * Given a PHP value and a GraphQL type, determine if the value will be
      * accepted for that type. This is primarily useful for validating the
@@ -155,14 +187,10 @@ class Values
     {
         // A value must be provided if the type is non-null.
         if ($type instanceof NonNull) {
-            $ofType = $type->getWrappedType();
             if (null === $value) {
-                if ($ofType->name) {
-                    return [ "Expected \"{$ofType->name}!\", found null." ];
-                }
-                return [ 'Expected non-null value, found null.' ];
+                return ['Expected "' . Utils::printSafe($type) . '", found null.'];
             }
-            return self::isValidPHPValue($value, $ofType);
+            return self::isValidPHPValue($value, $type->getWrappedType());
         }
 
         if (null === $value) {
@@ -235,9 +263,16 @@ class Values
      */
     private static function coerceValue(Type $type, $value)
     {
+        $undefined = Utils::undefined();
+        if ($value === $undefined) {
+            return $undefined;
+        }
+
         if ($type instanceof NonNull) {
-            // Note: we're not checking that the result of coerceValue is non-null.
-            // We only call this function after calling isValidPHPValue.
+            if ($value === null) {
+                // Intentionally return no value.
+                return $undefined;
+            }
             return self::coerceValue($type->getWrappedType(), $value);
         }
 
@@ -248,32 +283,57 @@ class Values
         if ($type instanceof ListOfType) {
             $itemType = $type->getWrappedType();
             if (is_array($value) || $value instanceof \Traversable) {
-                return Utils::map($value, function($item) use ($itemType) {
-                    return Values::coerceValue($itemType, $item);
-                });
+                $coercedValues = [];
+                foreach ($value as $item) {
+                    $itemValue = self::coerceValue($itemType, $item);
+                    if ($undefined === $itemValue) {
+                        // Intentionally return no value.
+                        return $undefined;
+                    }
+                    $coercedValues[] = $itemValue;
+                }
+                return $coercedValues;
             } else {
-                return [self::coerceValue($itemType, $value)];
+                $coercedValue = self::coerceValue($itemType, $value);
+                if ($coercedValue === $undefined) {
+                    // Intentionally return no value.
+                    return $undefined;
+                }
+                return [$coercedValue];
             }
         }
 
         if ($type instanceof InputObjectType) {
+            $coercedObj = [];
             $fields = $type->getFields();
-            $obj = [];
             foreach ($fields as $fieldName => $field) {
-                $fieldValue = self::coerceValue($field->getType(), isset($value[$fieldName]) ? $value[$fieldName] : null);
-                if (null === $fieldValue) {
-                    $fieldValue = $field->defaultValue;
+                if (!array_key_exists($fieldName, $value)) {
+                    if ($field->defaultValueExists()) {
+                        $coercedObj[$fieldName] = $field->defaultValue;
+                    } else if ($field->getType() instanceof NonNull) {
+                        // Intentionally return no value.
+                        return $undefined;
+                    }
+                    continue;
                 }
-                if (null !== $fieldValue) {
-                    $obj[$fieldName] = $fieldValue;
+                $fieldValue = self::coerceValue($field->getType(), $value[$fieldName]);
+                if ($fieldValue === $undefined) {
+                    // Intentionally return no value.
+                    return $undefined;
                 }
+                $coercedObj[$fieldName] = $fieldValue;
             }
-            return $obj;
-
+            return $coercedObj;
         }
 
         if ($type instanceof LeafType) {
-            return $type->parseValue($value);
+            $parsed = $type->parseValue($value);
+            if (null === $parsed) {
+                // null or invalid values represent a failure to parse correctly,
+                // in which case no value is returned.
+                return $undefined;
+            }
+            return $parsed;
         }
 
         throw new InvariantViolation('Must be input type');
diff --git a/src/Language/AST/NullValue.php b/src/Language/AST/NullValue.php
index e700bed..1b7f423 100644
--- a/src/Language/AST/NullValue.php
+++ b/src/Language/AST/NullValue.php
@@ -4,10 +4,4 @@ namespace GraphQL\Language\AST;
 class NullValue extends Node implements Value
 {
     public $kind = NodeType::NULL;
-
-    public static function getNullValue()
-    {
-        static $nullValue;
-        return $nullValue ?: $nullValue = new \stdClass();
-    }
 }
diff --git a/src/Utils.php b/src/Utils.php
index 17f32f0..903ffc9 100644
--- a/src/Utils.php
+++ b/src/Utils.php
@@ -9,6 +9,12 @@ use \Traversable, \InvalidArgumentException;
 
 class Utils
 {
+    public static function undefined()
+    {
+        static $undefined;
+        return $undefined ?: $undefined = new \stdClass();
+    }
+
     /**
      * @param object $obj
      * @param array  $vars
@@ -231,11 +237,7 @@ class Utils
     public static function printSafe($var)
     {
         if ($var instanceof Type) {
-            // FIXME: Replace with schema printer call
-            if ($var instanceof WrappingType) {
-                $var = $var->getWrappedType(true);
-            }
-            return $var->name;
+            return $var->toString();
         }
         if (is_object($var)) {
             return 'instance of ' . get_class($var);
diff --git a/src/Utils/AST.php b/src/Utils/AST.php
index 190e6d2..30183c0 100644
--- a/src/Utils/AST.php
+++ b/src/Utils/AST.php
@@ -4,6 +4,7 @@ namespace GraphQL\Utils;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\BooleanValue;
 use GraphQL\Language\AST\EnumValue;
+use GraphQL\Language\AST\Field;
 use GraphQL\Language\AST\FloatValue;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\ListValue;
@@ -12,6 +13,7 @@ use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\ObjectField;
 use GraphQL\Language\AST\ObjectValue;
 use GraphQL\Language\AST\StringValue;
+use GraphQL\Language\AST\Value;
 use GraphQL\Language\AST\Variable;
 use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\IDType;
@@ -178,6 +180,9 @@ class AST
      * A GraphQL type must be provided, which will be used to interpret different
      * GraphQL Value literals.
      *
+     * Returns `null` when the value could not be validly coerced according to
+     * the provided type.
+     *
      * | GraphQL Value        | PHP Value     |
      * | -------------------- | ------------- |
      * | Input Object         | Assoc Array   |
@@ -186,7 +191,7 @@ class AST
      * | String               | String        |
      * | Int / Float          | Int / Float   |
      * | Enum Value           | Mixed         |
-     * | Null Value           | null          |
+     * | Null Value           | stdClass      | instance of NullValue::getNullValue()
      *
      * @param $valueAST
      * @param InputType $type
@@ -196,30 +201,33 @@ class AST
      */
     public static function valueFromAST($valueAST, InputType $type, $variables = null)
     {
-        if ($type instanceof NonNull) {
-            // Note: we're not checking that the result of valueFromAST is non-null.
-            // We're assuming that this query has been validated and the value used
-            // here is of the correct type.
-            return self::valueFromAST($valueAST, $type->getWrappedType(), $variables);
-        }
+        $undefined = Utils::undefined();
 
         if (!$valueAST) {
             // When there is no AST, then there is also no value.
             // Importantly, this is different from returning the GraphQL null value.
-            return ;
+            return $undefined;
+        }
+
+        if ($type instanceof NonNull) {
+            if ($valueAST instanceof NullValue) {
+                // Invalid: intentionally return no value.
+                return $undefined;
+            }
+            return self::valueFromAST($valueAST, $type->getWrappedType(), $variables);
         }
 
         if ($valueAST instanceof NullValue) {
             // This is explicitly returning the value null.
-            return NullValue::getNullValue();
+            return null;
         }
 
         if ($valueAST instanceof Variable) {
             $variableName = $valueAST->name->value;
 
-            if (!$variables || !isset($variables[$variableName])) {
+            if (!$variables || !array_key_exists($variableName, $variables)) {
                 // No valid return value.
-                return ;
+                return $undefined;
             }
             // Note: we're not doing any checking that this variable is correct. We're
             // assuming that this query has been validated and the variable usage here
@@ -229,51 +237,99 @@ class AST
 
         if ($type instanceof ListOfType) {
             $itemType = $type->getWrappedType();
-            $items = $valueAST instanceof ListValue ? $valueAST->values : [$valueAST];
-            $result = [];
-            foreach ($items as $itemAST) {
-                $value = self::valueFromAST($itemAST, $itemType, $variables);
-                if ($value === NullValue::getNullValue()) {
-                    $value = null;
+
+            if ($valueAST instanceof ListValue) {
+                $coercedValues = [];
+                $itemASTs = $valueAST->values;
+                foreach ($itemASTs as $itemAST) {
+                    if (self::isMissingVariable($itemAST, $variables)) {
+                        // If an array contains a missing variable, it is either coerced to
+                        // null or if the item type is non-null, it considered invalid.
+                        if ($itemType instanceof NonNull) {
+                            // Invalid: intentionally return no value.
+                            return $undefined;
+                        }
+                        $coercedValues[] = null;
+                    } else {
+                        $itemValue = self::valueFromAST($itemAST, $itemType, $variables);
+                        if ($undefined === $itemValue) {
+                            // Invalid: intentionally return no value.
+                            return $undefined;
+                        }
+                        $coercedValues[] = $itemValue;
+                    }
                 }
-                $result[] = $value;
+                return $coercedValues;
             }
-            return $result;
+            $coercedValue = self::valueFromAST($valueAST, $itemType, $variables);
+            if ($undefined === $coercedValue) {
+                // Invalid: intentionally return no value.
+                return $undefined;
+            }
+            return [$coercedValue];
         }
 
         if ($type instanceof InputObjectType) {
-            $fields = $type->getFields();
             if (!$valueAST instanceof ObjectValue) {
-                // No valid return value.
-                return ;
+                // Invalid: intentionally return no value.
+                return $undefined;
             }
+
+            $coercedObj = [];
+            $fields = $type->getFields();
             $fieldASTs = Utils::keyMap($valueAST->fields, function($field) {return $field->name->value;});
-            $values = [];
             foreach ($fields as $field) {
-                $fieldAST = isset($fieldASTs[$field->name]) ? $fieldASTs[$field->name] : null;
+                /** @var Value $fieldAST */
+                $fieldName = $field->name;
+                $fieldAST = isset($fieldASTs[$fieldName]) ? $fieldASTs[$fieldName] : null;
+
+                if (!$fieldAST || self::isMissingVariable($fieldAST->value, $variables)) {
+                    if ($field->defaultValueExists()) {
+                        $coercedObj[$fieldName] = $field->defaultValue;
+                    } else if ($field->getType() instanceof NonNull) {
+                        // Invalid: intentionally return no value.
+                        return $undefined;
+                    }
+                    continue ;
+                }
+
                 $fieldValue = self::valueFromAST($fieldAST ? $fieldAST->value : null, $field->getType(), $variables);
 
-                // If field is not in AST and defaultValue was never set for this field - do not include it in result
-                if (null === $fieldValue && null === $field->defaultValue && !$field->defaultValueExists()) {
-                    continue;
+                if ($undefined === $fieldValue) {
+                    // Invalid: intentionally return no value.
+                    return $undefined;
                 }
-
-                // Set Explicit null value or default value:
-                if (NullValue::getNullValue() === $fieldValue) {
-                    $fieldValue = null;
-                } else if (null === $fieldValue) {
-                    $fieldValue = $field->defaultValue;
-                }
-
-                $values[$field->name] =  $fieldValue;
+                $coercedObj[$fieldName] = $fieldValue;
             }
-            return $values;
+            return $coercedObj;
         }
 
         if ($type instanceof LeafType) {
-            return $type->parseLiteral($valueAST);
+            $parsed = $type->parseLiteral($valueAST);
+
+            if (null === $parsed) {
+                // null represent a failure to parse correctly,
+                // in which case no value is returned.
+                return $undefined;
+            }
+
+            return $parsed;
         }
 
         throw new InvariantViolation('Must be input type');
     }
+
+
+    /**
+     * Returns true if the provided valueAST is a variable which is not defined
+     * in the set of variables.
+     * @param $valueAST
+     * @param $variables
+     * @return bool
+     */
+    private static function isMissingVariable($valueAST, $variables)
+    {
+        return $valueAST instanceof Variable &&
+        (!$variables || !array_key_exists($valueAST->name->value, $variables));
+    }
 }
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index f61a440..02ee918 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -157,14 +157,10 @@ class DocumentValidator
     {
         // A value must be provided if the type is non-null.
         if ($type instanceof NonNull) {
-            $wrappedType = $type->getWrappedType();
             if (!$valueAST || $valueAST instanceof NullValue) {
-                if ($wrappedType->name) {
-                    return [ "Expected \"{$wrappedType->name}!\", found null." ];
-                }
-                return ['Expected non-null value, found null.'];
+                return [ 'Expected "' . Utils::printSafe($type) . '", found null.' ];
             }
-            return static::isValidLiteralValue($wrappedType, $valueAST);
+            return static::isValidLiteralValue($type->getWrappedType(), $valueAST);
         }
 
         if (!$valueAST || $valueAST instanceof NullValue) {
diff --git a/src/Validator/Rules/QueryComplexity.php b/src/Validator/Rules/QueryComplexity.php
index 9646897..5ef6055 100644
--- a/src/Validator/Rules/QueryComplexity.php
+++ b/src/Validator/Rules/QueryComplexity.php
@@ -199,7 +199,7 @@ class QueryComplexity extends AbstractQuerySecurity
                 $this->variableDefs,
                 $rawVariableValues
             );
-            $args = Values::getArgumentValues($fieldDef->args, $node->arguments, $variableValues);
+            $args = Values::getArgumentValues($fieldDef, $node, $variableValues);
         }
 
         return $args;
diff --git a/tests/Executor/AbstractTest.php b/tests/Executor/AbstractTest.php
index 13df495..9babcb9 100644
--- a/tests/Executor/AbstractTest.php
+++ b/tests/Executor/AbstractTest.php
@@ -251,12 +251,11 @@ class AbstractTest extends \PHPUnit_Framework_TestCase
                     null
                 ]
             ],
-            'errors' => [
-                FormattedError::create(
-                    'Runtime Object type "Human" is not a possible type for "Pet".',
-                    [new SourceLocation(2, 11)]
-                )
-            ]
+            'errors' => [[
+                'message' => 'Runtime Object type "Human" is not a possible type for "Pet".',
+                'locations' => [['line' => 2, 'column' => 11]],
+                'path' => ['pets', 2]
+            ]]
         ];
         $actual = GraphQL::execute($schema, $query);
 
@@ -349,12 +348,11 @@ class AbstractTest extends \PHPUnit_Framework_TestCase
                     null
                 ]
             ],
-            'errors' => [
-                FormattedError::create(
-                    'Runtime Object type "Human" is not a possible type for "Pet".',
-                    [new SourceLocation(2, 11)]
-                )
-            ]
+            'errors' => [[
+                'message' => 'Runtime Object type "Human" is not a possible type for "Pet".',
+                'locations' => [['line' => 2, 'column' => 11]],
+                'path' => ['pets', 2]
+            ]]
         ];
         $this->assertEquals($expected, $result);
     }
diff --git a/tests/Executor/VariablesTest.php b/tests/Executor/VariablesTest.php
index cb8cdb9..1a2b9c5 100644
--- a/tests/Executor/VariablesTest.php
+++ b/tests/Executor/VariablesTest.php
@@ -81,19 +81,24 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
         $result = Executor::execute($this->schema(), $ast)->toArray();
 
         $expected = [
-            'data' => ['fieldWithObjectInput' => null]
+            'data' => ['fieldWithObjectInput' => null],
+            'errors' => [[
+                'message' => 'Argument "input" got invalid value ["foo", "bar", "baz"].' . "\n" .
+                    'Expected "TestInputObject", found not an object.',
+                'path' => ['fieldWithObjectInput']
+            ]]
         ];
-        $this->assertEquals($expected, $result);
+        $this->assertArraySubset($expected, $result);
 
         // properly runs parseLiteral on complex scalar types
         $doc = '
         {
-          fieldWithObjectInput(input: {a: "foo", d: "SerializedValue"})
+          fieldWithObjectInput(input: {c: "foo", d: "SerializedValue"})
         }
         ';
         $ast = Parser::parse($doc);
         $this->assertEquals(
-            ['data' => ['fieldWithObjectInput' => '{"a":"foo","d":"DeserializedValue"}']],
+            ['data' => ['fieldWithObjectInput' => '{"c":"foo","d":"DeserializedValue"}']],
             Executor::execute($this->schema(), $ast)->toArray()
         );
     }
@@ -332,6 +337,24 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
 
     // Describe: Handles non-nullable scalars
 
+    /**
+     * @it allows non-nullable inputs to be omitted given a default
+     */
+    public function testAllowsNonNullableInputsToBeOmittedGivenADefault()
+    {
+        $doc = '
+        query SetsNonNullable($value: String = "default") {
+          fieldWithNonNullableStringInput(input: $value)
+        }
+        ';
+        $ast = Parser::parse($doc);
+        $expected = [
+            'data' => ['fieldWithNonNullableStringInput' => '"default"']
+        ];
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
+
+    }
+
     /**
      * @it does not allow non-nullable inputs to be omitted in a variable
      */
@@ -368,13 +391,15 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
         $ast = Parser::parse($doc);
 
         try {
-            Executor::execute($this->schema(), $ast, null, ['value' => null]);
+            Executor::execute($this->schema(), $ast, null, null, ['value' => null]);
             $this->fail('Expected exception not thrown');
         } catch (Error $e) {
-            $expected = FormattedError::create(
-                'Variable "$value" of required type "String!" was not provided.',
-                [new SourceLocation(2, 31)]
-            );
+            $expected = [
+                'message' =>
+                    'Variable "$value" got invalid value null.' . "\n".
+                    'Expected "String!", found null.',
+                'locations' => [['line' => 2, 'column' => 31]]
+            ];
             $this->assertEquals($expected, Error::formatError($e));
         }
     }
@@ -411,9 +436,9 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
     }
 
     /**
-     * @it passes along null for non-nullable inputs if explcitly set in the query
+     * @it reports error for missing non-nullable inputs
      */
-    public function testPassesAlongNullForNonNullableInputsIfExplcitlySetInTheQuery()
+    public function testReportsErrorForMissingNonNullableInputs()
     {
         $doc = '
       {
@@ -421,7 +446,44 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
       }
         ';
         $ast = Parser::parse($doc);
-        $expected = ['data' => ['fieldWithNonNullableStringInput' => null]];
+        $expected = [
+            'data' => ['fieldWithNonNullableStringInput' => null],
+            'errors' => [[
+                'message' => 'Argument "input" of required type "String!" was not provided.',
+                'locations' => [ [ 'line' => 3, 'column' => 9 ] ],
+                'path' => [ 'fieldWithNonNullableStringInput' ]
+            ]]
+        ];
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
+    }
+
+    /**
+     * @it reports error for non-provided variables for non-nullable inputs
+     */
+    public function testReportsErrorForNonProvidedVariablesForNonNullableInputs()
+    {
+        // Note: this test would typically fail validation before encountering
+        // this execution error, however for queries which previously validated
+        // and are being run against a new schema which have introduced a breaking
+        // change to make a formerly non-required argument required, this asserts
+        // failure before allowing the underlying code to receive a non-null value.
+        $doc = '
+      {
+        fieldWithNonNullableStringInput(input: $foo)
+      }
+        ';
+        $ast = Parser::parse($doc);
+
+        $expected = [
+            'data' => ['fieldWithNonNullableStringInput' => null],
+            'errors' => [[
+                'message' =>
+                    'Argument "input" of required type "String!" was provided the ' .
+                    'variable "$foo" which was not provided a runtime value.',
+                'locations' => [['line' => 3, 'column' => 48]],
+                'path' => ['fieldWithNonNullableStringInput']
+            ]]
+        ];
         $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
     }
 
@@ -485,7 +547,8 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = FormattedError::create(
-            'Variable "$input" of required type "[String]!" was not provided.',
+            'Variable "$input" got invalid value null.' . "\n" .
+            'Expected "[String]!", found null.',
             [new SourceLocation(2, 17)]
         );
 
@@ -596,7 +659,8 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
         ';
         $ast = Parser::parse($doc);
         $expected = FormattedError::create(
-            'Variable "$input" of required type "[String!]!" was not provided.',
+            'Variable "$input" got invalid value null.' . "\n" .
+            'Expected "[String!]!", found null.',
             [new SourceLocation(2, 17)]
         );
         try {
@@ -663,11 +727,12 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
             Executor::execute($this->schema(), $ast, null, null, $vars);
             $this->fail('Expected exception not thrown');
         } catch (Error $error) {
-            $expected = FormattedError::create(
-                'Variable "$input" expected value of type "TestType!" which cannot ' .
-                'be used as an input type.',
-                [new SourceLocation(2, 17)]
-            );
+            $expected = [
+                'message' =>
+                    'Variable "$input" expected value of type "TestType!" which cannot ' .
+                    'be used as an input type.',
+                'locations' => [['line' => 2, 'column' => 25]]
+            ];
             $this->assertEquals($expected, Error::formatError($error));
         }
     }
@@ -692,7 +757,7 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
             $expected = FormattedError::create(
                 'Variable "$input" expected value of type "UnknownType!" which ' .
                 'cannot be used as an input type.',
-                [new SourceLocation(2, 17)]
+                [new SourceLocation(2, 25)]
             );
             $this->assertEquals($expected, Error::formatError($error));
         }
@@ -715,9 +780,9 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
     }
 
     /**
-     * @it when nullable variable provided
+     * @it when omitted variable provided
      */
-    public function testWhenNullableVariableProvided()
+    public function testWhenOmittedVariableProvided()
     {
         $ast = Parser::parse('query optionalVariable($optional: String) {
             fieldWithDefaultArgumentValue(input: $optional)
@@ -730,16 +795,27 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
     }
 
     /**
-     * @it when argument provided cannot be parsed
+     * @it not when argument cannot be coerced
      */
-    public function testWhenArgumentProvidedCannotBeParsed()
+    public function testNotWhenArgumentCannotBeCoerced()
     {
         $ast = Parser::parse('{
             fieldWithDefaultArgumentValue(input: WRONG_TYPE)
         }');
 
+        $expected = [
+            'data' => ['fieldWithDefaultArgumentValue' => null],
+            'errors' => [[
+                'message' =>
+                    'Argument "input" got invalid value WRONG_TYPE.' . "\n" .
+                    'Expected type "String", found WRONG_TYPE.',
+                'locations' => [ [ 'line' => 2, 'column' => 50 ] ],
+                'path' => [ 'fieldWithDefaultArgumentValue' ]
+            ]]
+        ];
+
         $this->assertEquals(
-            ['data' => ['fieldWithDefaultArgumentValue' => '"Hello World"']],
+            $expected,
             Executor::execute($this->schema(), $ast)->toArray()
         );
     }
diff --git a/tests/Utils/ValueFromAstTest.php b/tests/Utils/ValueFromAstTest.php
new file mode 100644
index 0000000..15799da
--- /dev/null
+++ b/tests/Utils/ValueFromAstTest.php
@@ -0,0 +1,244 @@
+<?php
+namespace GraphQL\Tests\Utils;
+
+use GraphQL\Language\AST\NullValue;
+use GraphQL\Language\Parser;
+use GraphQL\Type\Definition\EnumType;
+use GraphQL\Type\Definition\InputObjectType;
+use GraphQL\Type\Definition\Type;
+use GraphQL\Utils;
+use GraphQL\Utils\AST;
+
+class ValueFromAstTest extends \PHPUnit_Framework_TestCase
+{
+    private function runTestCase($type, $valueText, $expected)
+    {
+        $this->assertEquals($expected, AST::valueFromAST(Parser::parseValue($valueText), $type));
+    }
+
+    private function runTestCaseWithVars($variables, $type, $valueText, $expected)
+    {
+        $this->assertEquals($expected, AST::valueFromAST(Parser::parseValue($valueText), $type, $variables));
+    }
+
+    /**
+     * @it rejects empty input
+     */
+    public function testRejectsEmptyInput()
+    {
+        $this->assertEquals(Utils::undefined(), AST::valueFromAST(null, Type::boolean()));
+    }
+
+    /**
+     * @it converts according to input coercion rules
+     */
+    public function testConvertsAccordingToInputCoercionRules()
+    {
+        $this->runTestCase(Type::boolean(), 'true', true);
+        $this->runTestCase(Type::boolean(), 'false', false);
+        $this->runTestCase(Type::int(), '123', 123);
+        $this->runTestCase(Type::float(), '123', 123);
+        $this->runTestCase(Type::float(), '123.456', 123.456);
+        $this->runTestCase(Type::string(), '"abc123"', 'abc123');
+        $this->runTestCase(Type::id(), '123456', '123456');
+        $this->runTestCase(Type::id(), '"123456"', '123456');
+    }
+
+    /**
+     * @it does not convert when input coercion rules reject a value
+     */
+    public function testDoesNotConvertWhenInputCoercionRulesRejectAValue()
+    {
+        $undefined = Utils::undefined();
+
+        $this->runTestCase(Type::boolean(), '123', $undefined);
+        $this->runTestCase(Type::int(), '123.456', $undefined);
+        $this->runTestCase(Type::int(), 'true', $undefined);
+        $this->runTestCase(Type::int(), '"123"', $undefined);
+        $this->runTestCase(Type::float(), '"123"', $undefined);
+        $this->runTestCase(Type::string(), '123', $undefined);
+        $this->runTestCase(Type::string(), 'true', $undefined);
+        $this->runTestCase(Type::id(), '123.456', $undefined);
+    }
+
+    /**
+     * @it converts enum values according to input coercion rules
+     */
+    public function testConvertsEnumValuesAccordingToInputCoercionRules()
+    {
+        $testEnum = new EnumType([
+            'name' => 'TestColor',
+            'values' => [
+                'RED' => ['value' => 1],
+                'GREEN' => ['value' => 2],
+                'BLUE' => ['value' => 3],
+            ]
+        ]);
+
+        $this->runTestCase($testEnum, 'RED', 1);
+        $this->runTestCase($testEnum, 'BLUE', 3);
+        $this->runTestCase($testEnum, '3', Utils::undefined());
+        $this->runTestCase($testEnum, '"BLUE"', Utils::undefined());
+        $this->runTestCase($testEnum, 'null', null);
+    }
+
+    /**
+     * @it coerces to null unless non-null
+     */
+    public function testCoercesToNullUnlessNonNull()
+    {
+        $this->runTestCase(Type::boolean(), 'null', null);
+        $this->runTestCase(Type::nonNull(Type::boolean()), 'null', Utils::undefined());
+    }
+
+    /**
+     * @it coerces lists of values
+     */
+    public function testCoercesListsOfValues()
+    {
+        $listOfBool = Type::listOf(Type::boolean());
+        $undefined = Utils::undefined();
+
+        $this->runTestCase($listOfBool, 'true', [ true ]);
+        $this->runTestCase($listOfBool, '123', $undefined);
+        $this->runTestCase($listOfBool, 'null', null);
+        $this->runTestCase($listOfBool, '[true, false]', [ true, false ]);
+        $this->runTestCase($listOfBool, '[true, 123]', $undefined);
+        $this->runTestCase($listOfBool, '[true, null]', [ true, null ]);
+        $this->runTestCase($listOfBool, '{ true: true }', $undefined);
+    }
+
+    /**
+     * @it coerces non-null lists of values
+     */
+    public function testCoercesNonNullListsOfValues()
+    {
+        $nonNullListOfBool = Type::nonNull(Type::listOf(Type::boolean()));
+        $undefined = Utils::undefined();
+
+        $this->runTestCase($nonNullListOfBool, 'true', [ true ]);
+        $this->runTestCase($nonNullListOfBool, '123', $undefined);
+        $this->runTestCase($nonNullListOfBool, 'null', $undefined);
+        $this->runTestCase($nonNullListOfBool, '[true, false]', [ true, false ]);
+        $this->runTestCase($nonNullListOfBool, '[true, 123]', $undefined);
+        $this->runTestCase($nonNullListOfBool, '[true, null]', [ true, null ]);
+    }
+
+    /**
+     * @it coerces lists of non-null values
+     */
+    public function testCoercesListsOfNonNullValues()
+    {
+        $listOfNonNullBool = Type::listOf(Type::nonNull(Type::boolean()));
+        $undefined = Utils::undefined();
+
+        $this->runTestCase($listOfNonNullBool, 'true', [ true ]);
+        $this->runTestCase($listOfNonNullBool, '123', $undefined);
+        $this->runTestCase($listOfNonNullBool, 'null', null);
+        $this->runTestCase($listOfNonNullBool, '[true, false]', [ true, false ]);
+        $this->runTestCase($listOfNonNullBool, '[true, 123]', $undefined);
+        $this->runTestCase($listOfNonNullBool, '[true, null]', $undefined);
+    }
+
+    /**
+     * @it coerces non-null lists of non-null values
+     */
+    public function testCoercesNonNullListsOfNonNullValues()
+    {
+        $nonNullListOfNonNullBool = Type::nonNull(Type::listOf(Type::nonNull(Type::boolean())));
+        $undefined = Utils::undefined();
+
+        $this->runTestCase($nonNullListOfNonNullBool, 'true', [ true ]);
+        $this->runTestCase($nonNullListOfNonNullBool, '123', $undefined);
+        $this->runTestCase($nonNullListOfNonNullBool, 'null', $undefined);
+        $this->runTestCase($nonNullListOfNonNullBool, '[true, false]', [ true, false ]);
+        $this->runTestCase($nonNullListOfNonNullBool, '[true, 123]', $undefined);
+        $this->runTestCase($nonNullListOfNonNullBool, '[true, null]', $undefined);
+    }
+
+    private $inputObj;
+
+    private function inputObj()
+    {
+        return $this->inputObj ?: $this->inputObj = new InputObjectType([
+            'name' => 'TestInput',
+            'fields' => [
+                'int' => [ 'type' => Type::int(), 'defaultValue' => 42 ],
+                'bool' => [ 'type' => Type::boolean() ],
+                'requiredBool' => [ 'type' => Type::nonNull(Type::boolean()) ],
+            ]
+        ]);
+    }
+
+    /**
+     * @it coerces input objects according to input coercion rules
+     */
+    public function testCoercesInputObjectsAccordingToInputCoercionRules()
+    {
+        $testInputObj = $this->inputObj();
+        $undefined = Utils::undefined();
+
+        $this->runTestCase($testInputObj, 'null', null);
+        $this->runTestCase($testInputObj, '123', $undefined);
+        $this->runTestCase($testInputObj, '[]', $undefined);
+        $this->runTestCase($testInputObj, '{ int: 123, requiredBool: false }', ['int' => 123, 'requiredBool' => false]);
+        $this->runTestCase($testInputObj, '{ bool: true, requiredBool: false }', [ 'int' => 42, 'bool' => true, 'requiredBool' => false ]);
+        $this->runTestCase($testInputObj, '{ int: true, requiredBool: true }', $undefined);
+        $this->runTestCase($testInputObj, '{ requiredBool: null }', $undefined);
+        $this->runTestCase($testInputObj, '{ bool: true }', $undefined);
+    }
+
+    /**
+     * @it accepts variable values assuming already coerced
+     */
+    public function testAcceptsVariableValuesAssumingAlreadyCoerced()
+    {
+        $this->runTestCaseWithVars([], Type::boolean(), '$var', Utils::undefined());
+        $this->runTestCaseWithVars([ 'var' => true ], Type::boolean(), '$var', true);
+        $this->runTestCaseWithVars([ 'var' => null ], Type::boolean(), '$var', null);
+    }
+
+    /**
+     * @it asserts variables are provided as items in lists
+     */
+    public function testAssertsVariablesAreProvidedAsItemsInLists()
+    {
+        $listOfBool = Type::listOf(Type::boolean());
+        $listOfNonNullBool = Type::listOf(Type::nonNull(Type::boolean()));
+
+        $this->runTestCaseWithVars([], $listOfBool, '[ $foo ]', [ null ]);
+        $this->runTestCaseWithVars([], $listOfNonNullBool, '[ $foo ]', Utils::undefined());
+        $this->runTestCaseWithVars([ 'foo' => true ], $listOfNonNullBool, '[ $foo ]', [ true ]);
+        // Note: variables are expected to have already been coerced, so we
+        // do not expect the singleton wrapping behavior for variables.
+        $this->runTestCaseWithVars([ 'foo' => true ], $listOfNonNullBool, '$foo', true);
+        $this->runTestCaseWithVars([ 'foo' => [ true ] ], $listOfNonNullBool, '$foo', [ true ]);
+    }
+
+    /**
+     * @it omits input object fields for unprovided variables
+     */
+    public function testOmitsInputObjectFieldsForUnprovidedVariables()
+    {
+        $testInputObj = $this->inputObj();
+
+        $this->runTestCaseWithVars(
+            [],
+            $testInputObj,
+            '{ int: $foo, bool: $foo, requiredBool: true }',
+            [ 'int' => 42, 'requiredBool' => true ]
+        );
+        $this->runTestCaseWithVars(
+            [],
+            $testInputObj,
+            '{ requiredBool: $foo }',
+            Utils::undefined()
+        );
+        $this->runTestCaseWithVars(
+            [ 'foo' => true ],
+            $testInputObj,
+            '{ requiredBool: $foo }',
+            [ 'int' => 42, 'requiredBool' => true ]
+        );
+    }
+}