diff --git a/src/Executor/Values.php b/src/Executor/Values.php
index 0efe9b2..18bba17 100644
--- a/src/Executor/Values.php
+++ b/src/Executor/Values.php
@@ -5,17 +5,16 @@ namespace GraphQL\Executor;
 use GraphQL\Error\Error;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\Argument;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\VariableDefinition;
 use GraphQL\Language\Printer;
 use GraphQL\Schema;
-use GraphQL\Type\Definition\EnumType;
 use GraphQL\Type\Definition\FieldArgument;
 use GraphQL\Type\Definition\InputObjectType;
 use GraphQL\Type\Definition\InputType;
 use GraphQL\Type\Definition\LeafType;
 use GraphQL\Type\Definition\ListOfType;
 use GraphQL\Type\Definition\NonNull;
-use GraphQL\Type\Definition\ScalarType;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Utils;
 
@@ -65,16 +64,29 @@ class Values
             $valueAST = isset($argASTMap[$name]) ? $argASTMap[$name]->value : null;
             $value = Utils\AST::valueFromAST($valueAST, $argDef->getType(), $variableValues);
 
+            if (null === $value && null === $argDef->defaultValue && !$argDef->defaultValueExists()) {
+                continue;
+            }
+
             if (null === $value) {
                 $value = $argDef->defaultValue;
             }
-            if (null !== $value) {
-                $result[$name] = $value;
+            if (NullValue::getNullValue() === $value) {
+                $value = null;
             }
+            $result[$name] = $value;
         }
         return $result;
     }
 
+    /**
+     * @deprecated Moved to Utils\AST::valueFromAST
+     *
+     * @param $valueAST
+     * @param InputType $type
+     * @param null $variables
+     * @return array|null|\stdClass
+     */
     public static function valueFromAST($valueAST, InputType $type, $variables = null)
     {
         return Utils\AST::valueFromAST($valueAST, $type, $variables);
@@ -105,7 +117,8 @@ class Values
             if (null === $input) {
                 $defaultValue = $definitionAST->defaultValue;
                 if ($defaultValue) {
-                    return Utils\AST::valueFromAST($defaultValue, $inputType);
+                    $value = Utils\AST::valueFromAST($defaultValue, $inputType);
+                    return $value === NullValue::getNullValue() ? null : $value;
                 }
             }
             return self::coerceValue($inputType, $input);
diff --git a/src/Language/AST/NodeType.php b/src/Language/AST/NodeType.php
index 4b73d8e..42886a7 100644
--- a/src/Language/AST/NodeType.php
+++ b/src/Language/AST/NodeType.php
@@ -31,6 +31,7 @@ class NodeType
     const STRING = 'StringValue';
     const BOOLEAN = 'BooleanValue';
     const ENUM = 'EnumValue';
+    const NULL = 'NullValue';
     const LST = 'ListValue';
     const OBJECT = 'ObjectValue';
     const OBJECT_FIELD = 'ObjectField';
diff --git a/src/Language/AST/NullValue.php b/src/Language/AST/NullValue.php
new file mode 100644
index 0000000..e700bed
--- /dev/null
+++ b/src/Language/AST/NullValue.php
@@ -0,0 +1,13 @@
+<?php
+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/Language/Parser.php b/src/Language/Parser.php
index 795d9ea..00cd447 100644
--- a/src/Language/Parser.php
+++ b/src/Language/Parser.php
@@ -27,6 +27,7 @@ use GraphQL\Language\AST\Location;
 use GraphQL\Language\AST\Name;
 use GraphQL\Language\AST\NamedType;
 use GraphQL\Language\AST\NonNullType;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\ObjectField;
 use GraphQL\Language\AST\ObjectTypeDefinition;
 use GraphQL\Language\AST\ObjectValue;
@@ -610,16 +611,19 @@ class Parser
      *   - FloatValue
      *   - StringValue
      *   - BooleanValue
+     *   - NullValue
      *   - EnumValue
      *   - ListValue[?Const]
      *   - ObjectValue[?Const]
      *
      * BooleanValue : one of `true` `false`
      *
+     * NullValue : `null`
+     *
      * EnumValue : Name but not `true`, `false` or `null`
      *
      * @param $isConst
-     * @return BooleanValue|EnumValue|FloatValue|IntValue|StringValue|Variable|ListValue|ObjectValue
+     * @return BooleanValue|EnumValue|FloatValue|IntValue|StringValue|Variable|ListValue|ObjectValue|NullValue
      * @throws SyntaxError
      */
     function parseValueLiteral($isConst)
@@ -655,7 +659,12 @@ class Parser
                         'value' => $token->value === 'true',
                         'loc' => $this->loc($token)
                     ]);
-                } else if ($token->value !== 'null') {
+                } else if ($token->value === 'null') {
+                    $this->lexer->advance();
+                    return new NullValue([
+                        'loc' => $this->loc($token)
+                    ]);
+                } else {
                     $this->lexer->advance();
                     return new EnumValue([
                         'value' => $token->value,
diff --git a/src/Language/Printer.php b/src/Language/Printer.php
index b42d88b..1595ae3 100644
--- a/src/Language/Printer.php
+++ b/src/Language/Printer.php
@@ -21,11 +21,11 @@ use GraphQL\Language\AST\FragmentSpread;
 use GraphQL\Language\AST\InlineFragment;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\ListType;
-use GraphQL\Language\AST\Name;
 use GraphQL\Language\AST\NamedType;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\NodeType;
 use GraphQL\Language\AST\NonNullType;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\ObjectField;
 use GraphQL\Language\AST\ObjectTypeDefinition;
 use GraphQL\Language\AST\ObjectValue;
@@ -112,14 +112,33 @@ class Printer
                 },
 
                 // Value
-                NodeType::INT => function(IntValue  $node) {return $node->value;},
-                NodeType::FLOAT => function(FloatValue $node) {return $node->value;},
-                NodeType::STRING => function(StringValue $node) {return json_encode($node->value);},
-                NodeType::BOOLEAN => function(BooleanValue $node) {return $node->value ? 'true' : 'false';},
-                NodeType::ENUM => function(EnumValue $node) {return $node->value;},
-                NodeType::LST => function(ListValue $node) {return '[' . $this->join($node->values, ', ') . ']';},
-                NodeType::OBJECT => function(ObjectValue $node) {return '{' . $this->join($node->fields, ', ') . '}';},
-                NodeType::OBJECT_FIELD => function(ObjectField $node) {return $node->name . ': ' . $node->value;},
+                NodeType::INT => function(IntValue  $node) {
+                    return $node->value;
+                },
+                NodeType::FLOAT => function(FloatValue $node) {
+                    return $node->value;
+                },
+                NodeType::STRING => function(StringValue $node) {
+                    return json_encode($node->value);
+                },
+                NodeType::BOOLEAN => function(BooleanValue $node) {
+                    return $node->value ? 'true' : 'false';
+                },
+                NodeType::NULL => function(NullValue $node) {
+                    return 'null';
+                },
+                NodeType::ENUM => function(EnumValue $node) {
+                    return $node->value;
+                },
+                NodeType::LST => function(ListValue $node) {
+                    return '[' . $this->join($node->values, ', ') . ']';
+                },
+                NodeType::OBJECT => function(ObjectValue $node) {
+                    return '{' . $this->join($node->fields, ', ') . '}';
+                },
+                NodeType::OBJECT_FIELD => function(ObjectField $node) {
+                    return $node->name . ': ' . $node->value;
+                },
 
                 // Directive
                 NodeType::DIRECTIVE => function(Directive $node) {
@@ -127,9 +146,15 @@ class Printer
                 },
 
                 // Type
-                NodeType::NAMED_TYPE => function(NamedType $node) {return $node->name;},
-                NodeType::LIST_TYPE => function(ListType $node) {return '[' . $node->type . ']';},
-                NodeType::NON_NULL_TYPE => function(NonNullType $node) {return $node->type . '!';},
+                NodeType::NAMED_TYPE => function(NamedType $node) {
+                    return $node->name;
+                },
+                NodeType::LIST_TYPE => function(ListType $node) {
+                    return '[' . $node->type . ']';
+                },
+                NodeType::NON_NULL_TYPE => function(NonNullType $node) {
+                    return $node->type . '!';
+                },
 
                 // Type System Definitions
                 NodeType::SCHEMA_DEFINITION => function(SchemaDefinition $def) {
@@ -139,7 +164,9 @@ class Printer
                         $this->block($def->operationTypes)
                     ], ' ');
                 },
-                NodeType::OPERATION_TYPE_DEFINITION => function(OperationTypeDefinition $def) {return $def->operation . ': ' . $def->type;},
+                NodeType::OPERATION_TYPE_DEFINITION => function(OperationTypeDefinition $def) {
+                    return $def->operation . ': ' . $def->type;
+                },
 
                 NodeType::SCALAR_TYPE_DEFINITION => function(ScalarTypeDefinition $def) {
                     return $this->join(['scalar', $def->name, $this->join($def->directives, ' ')], ' ');
diff --git a/src/Language/Visitor.php b/src/Language/Visitor.php
index 589e720..5a69c9d 100644
--- a/src/Language/Visitor.php
+++ b/src/Language/Visitor.php
@@ -65,6 +65,7 @@ class Visitor
         NodeType::FLOAT => [],
         NodeType::STRING => [],
         NodeType::BOOLEAN => [],
+        NodeType::NULL => [],
         NodeType::ENUM => [],
         NodeType::LST => ['values'],
         NodeType::OBJECT => ['fields'],
diff --git a/src/Type/Definition/FieldArgument.php b/src/Type/Definition/FieldArgument.php
index 8385b06..7ce1b21 100644
--- a/src/Type/Definition/FieldArgument.php
+++ b/src/Type/Definition/FieldArgument.php
@@ -40,6 +40,11 @@ class FieldArgument
      */
     private $resolvedType;
 
+    /**
+     * @var bool
+     */
+    private $defaultValueExists = false;
+
     /**
      * @param array $config
      * @return array
@@ -62,17 +67,23 @@ class FieldArgument
      */
     public function __construct(array $def)
     {
-        $def += [
-            'type' => null,
-            'name' => null,
-            'defaultValue' => null,
-            'description' => null
-        ];
-
-        $this->type = $def['type'];
-        $this->name = $def['name'];
-        $this->description = $def['description'];
-        $this->defaultValue = $def['defaultValue'];
+        foreach ($def as $key => $value) {
+            switch ($key) {
+                case 'type':
+                    $this->type = $value;
+                    break;
+                case 'name':
+                    $this->name = $value;
+                    break;
+                case 'defaultValue':
+                    $this->defaultValue = $value;
+                    $this->defaultValueExists = true;
+                    break;
+                case 'description':
+                    $this->description = $value;
+                    break;
+            }
+        }
         $this->config = $def;
     }
 
@@ -87,4 +98,12 @@ class FieldArgument
         }
         return $this->resolvedType;
     }
+
+    /**
+     * @return bool
+     */
+    public function defaultValueExists()
+    {
+        return $this->defaultValueExists;
+    }
 }
diff --git a/src/Type/Definition/InputObjectField.php b/src/Type/Definition/InputObjectField.php
index aafe4bc..a60f951 100644
--- a/src/Type/Definition/InputObjectField.php
+++ b/src/Type/Definition/InputObjectField.php
@@ -27,6 +27,13 @@ class InputObjectField
      */
     public $type;
 
+    /**
+     * Helps to differentiate when `defaultValue` is `null` and when it was not even set initially
+     *
+     * @var bool
+     */
+    private $defaultValueExists = false;
+
     /**
      * InputObjectField constructor.
      * @param array $opts
@@ -34,7 +41,16 @@ class InputObjectField
     public function __construct(array $opts)
     {
         foreach ($opts as $k => $v) {
-            $this->{$k} = $v;
+            switch ($k) {
+                case 'defaultValue':
+                    $this->defaultValue = $v;
+                    $this->defaultValueExists = true;
+                    break;
+                case 'defaultValueExists':
+                    break;
+                default:
+                    $this->{$k} = $v;
+            }
         }
     }
 
@@ -45,4 +61,12 @@ class InputObjectField
     {
         return Type::resolve($this->type);
     }
+
+    /**
+     * @return bool
+     */
+    public function defaultValueExists()
+    {
+        return $this->defaultValueExists;
+    }
 }
diff --git a/src/Utils/AST.php b/src/Utils/AST.php
index b100a5b..190e6d2 100644
--- a/src/Utils/AST.php
+++ b/src/Utils/AST.php
@@ -8,6 +8,7 @@ use GraphQL\Language\AST\FloatValue;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\ListValue;
 use GraphQL\Language\AST\Name;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\ObjectField;
 use GraphQL\Language\AST\ObjectValue;
 use GraphQL\Language\AST\StringValue;
@@ -43,21 +44,24 @@ class AST
      * | Int           | Int                  |
      * | Float         | Int / Float          |
      * | Mixed         | Enum Value           |
+     * | null          | NullValue            |
      *
      * @param $value
      * @param InputType $type
-     * @return ObjectValue|ListValue|BooleanValue|IntValue|FloatValue|EnumValue|StringValue
+     * @return ObjectValue|ListValue|BooleanValue|IntValue|FloatValue|EnumValue|StringValue|NullValue
      */
     static function astFromValue($value, InputType $type)
     {
         if ($type instanceof NonNull) {
-            // Note: we're not checking that the result is non-null.
-            // This function is not responsible for validating the input value.
-            return self::astFromValue($value, $type->getWrappedType());
+            $astValue = self::astFromValue($value, $type->getWrappedType());
+            if ($astValue instanceof NullValue) {
+                return null;
+            }
+            return $astValue;
         }
 
         if ($value === null) {
-            return null;
+            return new NullValue([]);
         }
 
         // Convert PHP array to GraphQL list. If the GraphQLType is a list, but
@@ -80,7 +84,8 @@ class AST
         // Populate the fields of the input object by creating ASTs from each value
         // in the PHP object according to the fields in the input type.
         if ($type instanceof InputObjectType) {
-            $isArrayLike = is_array($value) || $value instanceof \ArrayAccess;
+            $isArray = is_array($value);
+            $isArrayLike = $isArray || $value instanceof \ArrayAccess;
             if ($value === null || (!$isArrayLike && !is_object($value))) {
                 return null;
             }
@@ -92,13 +97,29 @@ class AST
                 } else {
                     $fieldValue = isset($value->{$fieldName}) ? $value->{$fieldName} : null;
                 }
-                $fieldValue = self::astFromValue($fieldValue, $field->getType());
 
-                if ($fieldValue) {
-                    $fieldASTs[] = new ObjectField([
-                        'name' => new Name(['value' => $fieldName]),
-                        'value' => $fieldValue
-                    ]);
+                // Have to check additionally if key exists, since we differentiate between
+                // "no key" and "value is null":
+                if (null !== $fieldValue) {
+                    $fieldExists = true;
+                } else if ($isArray) {
+                    $fieldExists = array_key_exists($fieldName, $value);
+                } else if ($isArrayLike) {
+                    /** @var \ArrayAccess $value */
+                    $fieldExists = $value->offsetExists($fieldName);
+                } else {
+                    $fieldExists = property_exists($value, $fieldName);
+                }
+
+                if ($fieldExists) {
+                    $fieldNode = self::astFromValue($fieldValue, $field->getType());
+
+                    if ($fieldNode) {
+                        $fieldASTs[] = new ObjectField([
+                            'name' => new Name(['value' => $fieldName]),
+                            'value' => $fieldNode
+                        ]);
+                    }
                 }
             }
             return new ObjectValue(['fields' => $fieldASTs]);
@@ -165,11 +186,12 @@ class AST
      * | String               | String        |
      * | Int / Float          | Int / Float   |
      * | Enum Value           | Mixed         |
+     * | Null Value           | null          |
      *
      * @param $valueAST
      * @param InputType $type
      * @param null $variables
-     * @return array|null
+     * @return array|null|\stdClass
      * @throws \Exception
      */
     public static function valueFromAST($valueAST, InputType $type, $variables = null)
@@ -182,14 +204,22 @@ class AST
         }
 
         if (!$valueAST) {
-            return null;
+            // When there is no AST, then there is also no value.
+            // Importantly, this is different from returning the GraphQL null value.
+            return ;
+        }
+
+        if ($valueAST instanceof NullValue) {
+            // This is explicitly returning the value null.
+            return NullValue::getNullValue();
         }
 
         if ($valueAST instanceof Variable) {
             $variableName = $valueAST->name->value;
 
             if (!$variables || !isset($variables[$variableName])) {
-                return null;
+                // No valid return value.
+                return ;
             }
             // 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
@@ -199,19 +229,23 @@ class AST
 
         if ($type instanceof ListOfType) {
             $itemType = $type->getWrappedType();
-            if ($valueAST instanceof ListValue) {
-                return array_map(function($itemAST) use ($itemType, $variables) {
-                    return self::valueFromAST($itemAST, $itemType, $variables);
-                }, $valueAST->values);
-            } else {
-                return [self::valueFromAST($valueAST, $itemType, $variables)];
+            $items = $valueAST instanceof ListValue ? $valueAST->values : [$valueAST];
+            $result = [];
+            foreach ($items as $itemAST) {
+                $value = self::valueFromAST($itemAST, $itemType, $variables);
+                if ($value === NullValue::getNullValue()) {
+                    $value = null;
+                }
+                $result[] = $value;
             }
+            return $result;
         }
 
         if ($type instanceof InputObjectType) {
             $fields = $type->getFields();
             if (!$valueAST instanceof ObjectValue) {
-                return null;
+                // No valid return value.
+                return ;
             }
             $fieldASTs = Utils::keyMap($valueAST->fields, function($field) {return $field->name->value;});
             $values = [];
@@ -219,12 +253,19 @@ class AST
                 $fieldAST = isset($fieldASTs[$field->name]) ? $fieldASTs[$field->name] : null;
                 $fieldValue = self::valueFromAST($fieldAST ? $fieldAST->value : null, $field->getType(), $variables);
 
-                if (null === $fieldValue) {
+                // 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;
+                }
+
+                // Set Explicit null value or default value:
+                if (NullValue::getNullValue() === $fieldValue) {
+                    $fieldValue = null;
+                } else if (null === $fieldValue) {
                     $fieldValue = $field->defaultValue;
                 }
-                if (null !== $fieldValue) {
-                    $values[$field->name] = $fieldValue;
-                }
+
+                $values[$field->name] =  $fieldValue;
             }
             return $values;
         }
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index a890838..0fd0299 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -8,6 +8,7 @@ use GraphQL\Language\AST\Document;
 use GraphQL\Language\AST\FragmentSpread;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\NodeType;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\Value;
 use GraphQL\Language\AST\Variable;
 use GraphQL\Language\Printer;
@@ -155,7 +156,7 @@ class DocumentValidator
         // A value must be provided if the type is non-null.
         if ($type instanceof NonNull) {
             $wrappedType = $type->getWrappedType();
-            if (!$valueAST) {
+            if (!$valueAST || $valueAST instanceof NullValue) {
                 if ($wrappedType->name) {
                     return [ "Expected \"{$wrappedType->name}!\", found null." ];
                 }
@@ -164,7 +165,7 @@ class DocumentValidator
             return static::isValidLiteralValue($wrappedType, $valueAST);
         }
 
-        if (!$valueAST) {
+        if (!$valueAST || $valueAST instanceof NullValue) {
             return [];
         }
 
diff --git a/tests/Executor/ExecutorTest.php b/tests/Executor/ExecutorTest.php
index 07daaa3..12904b5 100644
--- a/tests/Executor/ExecutorTest.php
+++ b/tests/Executor/ExecutorTest.php
@@ -836,7 +836,7 @@ class ExecutorTest extends \PHPUnit_Framework_TestCase
         $result = Executor::execute($schema, $query);
         $expected = [
             'data' => [
-                'field' => '{"a":1,"c":0,"d":false,"e":"0","f":"some-string","h":{"a":1,"b":"test"}}'
+                'field' => '{"a":1,"b":null,"c":0,"d":false,"e":"0","f":"some-string","h":{"a":1,"b":"test"}}'
             ]
         ];
 
diff --git a/tests/Executor/VariablesTest.php b/tests/Executor/VariablesTest.php
index d71f9b7..cb8cdb9 100644
--- a/tests/Executor/VariablesTest.php
+++ b/tests/Executor/VariablesTest.php
@@ -49,6 +49,28 @@ class VariablesTest extends \PHPUnit_Framework_TestCase
 
         $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
 
+        // properly parses null value to null
+        $doc = '
+        {
+          fieldWithObjectInput(input: {a: null, b: null, c: "C", d: null})
+        }
+        ';
+        $ast = Parser::parse($doc);
+        $expected = ['data' => ['fieldWithObjectInput' => '{"a":null,"b":null,"c":"C","d":null}']];
+
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
+
+        // properly parses null value in list
+        $doc = '
+        {
+          fieldWithObjectInput(input: {b: ["A",null,"C"], c: "C"})
+        }
+        ';
+        $ast = Parser::parse($doc);
+        $expected = ['data' => ['fieldWithObjectInput' => '{"b":["A",null,"C"],"c":"C"}']];
+
+        $this->assertEquals($expected, Executor::execute($this->schema(), $ast)->toArray());
+
         // does not use incorrect value
         $doc = '
         {
diff --git a/tests/Language/ParserTest.php b/tests/Language/ParserTest.php
index 6977ba4..e4caa4d 100644
--- a/tests/Language/ParserTest.php
+++ b/tests/Language/ParserTest.php
@@ -6,6 +6,7 @@ use GraphQL\Language\AST\Field;
 use GraphQL\Language\AST\Name;
 use GraphQL\Language\AST\Node;
 use GraphQL\Language\AST\NodeType;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\SelectionSet;
 use GraphQL\Language\AST\StringValue;
 use GraphQL\Language\Parser;
@@ -106,15 +107,6 @@ fragment MissingOn Type
         Parser::parse('{ ...on }');
     }
 
-    /**
-     * @it does not allow null as value
-     */
-    public function testDoesNotAllowNullAsValue()
-    {
-        $this->setExpectedException('GraphQL\Error\SyntaxError', 'Syntax Error GraphQL (1:39) Unexpected Name "null"');
-        Parser::parse('{ fieldWithNullableStringInput(input: null) }');
-    }
-
     /**
      * @it parses multi-byte characters
      */
@@ -393,6 +385,17 @@ fragment $fragmentName on Type {
 
     // Describe: parseValue
 
+    /**
+     * @it parses null value
+     */
+    public function testParsesNullValues()
+    {
+        $this->assertEquals([
+            'kind' => NodeType::NULL,
+            'loc' => ['start' => 0, 'end' => 4]
+        ], $this->nodeToArray(Parser::parseValue('null')));
+    }
+
     /**
      * @it parses list values
      */
diff --git a/tests/Language/PrinterTest.php b/tests/Language/PrinterTest.php
index a73d55e..bf584ee 100644
--- a/tests/Language/PrinterTest.php
+++ b/tests/Language/PrinterTest.php
@@ -154,7 +154,7 @@ fragment frag on Friend {
 }
 
 {
-  unnamed(truthy: true, falsey: false)
+  unnamed(truthy: true, falsey: false, nullish: null)
   query
 }
 
diff --git a/tests/Language/SchemaPrinterTest.php b/tests/Language/SchemaPrinterTest.php
index c0ead91..e704ef4 100644
--- a/tests/Language/SchemaPrinterTest.php
+++ b/tests/Language/SchemaPrinterTest.php
@@ -63,6 +63,7 @@ type Foo implements Bar {
   four(argument: String = "string"): String
   five(argument: [String] = ["string", "string"]): String
   six(argument: InputType = {key: "value"}): Type
+  seven(argument: Int = null): Type
 }
 
 type AnnotatedObject @onObject(arg: "value") {
diff --git a/tests/Language/VisitorTest.php b/tests/Language/VisitorTest.php
index 2710702..fd5fd33 100644
--- a/tests/Language/VisitorTest.php
+++ b/tests/Language/VisitorTest.php
@@ -636,6 +636,12 @@ class VisitorTest extends \PHPUnit_Framework_TestCase
             [ 'enter', 'BooleanValue', 'value', 'Argument' ],
             [ 'leave', 'BooleanValue', 'value', 'Argument' ],
             [ 'leave', 'Argument', 1, null ],
+            [ 'enter', 'Argument', 2, null ],
+            [ 'enter', 'Name', 'name', 'Argument' ],
+            [ 'leave', 'Name', 'name', 'Argument' ],
+            [ 'enter', 'NullValue', 'value', 'Argument' ],
+            [ 'leave', 'NullValue', 'value', 'Argument' ],
+            [ 'leave', 'Argument', 2, null ],
             [ 'leave', 'Field', 0, null ],
             [ 'enter', 'Field', 1, null ],
             [ 'enter', 'Name', 'name', 'Field' ],
diff --git a/tests/Language/kitchen-sink.graphql b/tests/Language/kitchen-sink.graphql
index 0e04e2e..993de9a 100644
--- a/tests/Language/kitchen-sink.graphql
+++ b/tests/Language/kitchen-sink.graphql
@@ -52,6 +52,6 @@ fragment frag on Friend {
 }
 
 {
-  unnamed(truthy: true, falsey: false),
+  unnamed(truthy: true, falsey: false, nullish: null),
   query
 }
diff --git a/tests/Language/schema-kitchen-sink.graphql b/tests/Language/schema-kitchen-sink.graphql
index d148276..a56c0e4 100644
--- a/tests/Language/schema-kitchen-sink.graphql
+++ b/tests/Language/schema-kitchen-sink.graphql
@@ -17,6 +17,7 @@ type Foo implements Bar {
   four(argument: String = "string"): String
   five(argument: [String] = ["string", "string"]): String
   six(argument: InputType = {key: "value"}): Type
+  seven(argument: Int = null): Type
 }
 
 type AnnotatedObject @onObject(arg: "value") {
diff --git a/tests/Utils/AstFromValueTest.php b/tests/Utils/AstFromValueTest.php
index a65762a..6468fb1 100644
--- a/tests/Utils/AstFromValueTest.php
+++ b/tests/Utils/AstFromValueTest.php
@@ -7,6 +7,7 @@ use GraphQL\Language\AST\FloatValue;
 use GraphQL\Language\AST\IntValue;
 use GraphQL\Language\AST\ListValue;
 use GraphQL\Language\AST\Name;
+use GraphQL\Language\AST\NullValue;
 use GraphQL\Language\AST\ObjectField;
 use GraphQL\Language\AST\ObjectValue;
 use GraphQL\Language\AST\StringValue;
@@ -26,9 +27,11 @@ class ASTFromValueTest extends \PHPUnit_Framework_TestCase
     {
         $this->assertEquals(new BooleanValue(['value' => true]), AST::astFromValue(true, Type::boolean()));
         $this->assertEquals(new BooleanValue(['value' => false]), AST::astFromValue(false, Type::boolean()));
-        $this->assertEquals(null, AST::astFromValue(null, Type::boolean()));
+        $this->assertEquals(new NullValue([]), AST::astFromValue(null, Type::boolean()));
         $this->assertEquals(new BooleanValue(['value' => false]), AST::astFromValue(0, Type::boolean()));
         $this->assertEquals(new BooleanValue(['value' => true]), AST::astFromValue(1, Type::boolean()));
+        $this->assertEquals(new BooleanValue(['value' => false]), AST::astFromValue(0, Type::nonNull(Type::boolean())));
+        $this->assertEquals(null, AST::astFromValue(null, Type::nonNull(Type::boolean()))); // Note: null means that AST cannot
     }
 
     /**
@@ -70,7 +73,8 @@ class ASTFromValueTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(new StringValue(['value' => 'VA\\nLUE']), AST::astFromValue("VA\nLUE", Type::string()));
         $this->assertEquals(new StringValue(['value' => '123']), AST::astFromValue(123, Type::string()));
         $this->assertEquals(new StringValue(['value' => 'false']), AST::astFromValue(false, Type::string()));
-        $this->assertEquals(null, AST::astFromValue(null, Type::string()));
+        $this->assertEquals(new NullValue([]), AST::astFromValue(null, Type::string()));
+        $this->assertEquals(null, AST::astFromValue(null, Type::nonNull(Type::string())));
     }
 
     /**
@@ -83,7 +87,16 @@ class ASTFromValueTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals(new StringValue(['value' => 'VA\\nLUE']), AST::astFromValue("VA\nLUE", Type::id()));
         $this->assertEquals(new IntValue(['value' => '123']), AST::astFromValue(123, Type::id()));
         $this->assertEquals(new StringValue(['value' => 'false']), AST::astFromValue(false, Type::id()));
-        $this->assertEquals(null, AST::astFromValue(null, Type::id()));
+        $this->assertEquals(new NullValue([]), AST::astFromValue(null, Type::id()));
+        $this->assertEquals(null, AST::astFromValue(null, Type::nonNull(Type::id())));
+    }
+
+    /**
+     * @it does not converts NonNull values to NullValue
+     */
+    public function testDoesNotConvertsNonNullValuestoNullValue()
+    {
+        $this->assertSame(null, AST::astFromValue(null, Type::nonNull(Type::boolean())));
     }
 
     /**
@@ -156,6 +169,44 @@ class ASTFromValueTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expected, AST::astFromValue((object) $data, $inputObj));
     }
 
+    public function testConvertsInputObjectsWithExplicitNulls()
+    {
+        $inputObj = new InputObjectType([
+            'name' => 'MyInputObj',
+            'fields' => [
+                'foo' => Type::float(),
+                'bar' => $this->myEnum()
+            ]
+        ]);
+
+        $this->assertEquals(new ObjectValue([
+            'fields' => [
+                $this->objectField('foo', new NullValue([]))
+            ]
+        ]), AST::astFromValue(['foo' => null], $inputObj));
+/*
+    const inputObj = new GraphQLInputObjectType({
+      name: 'MyInputObj',
+      fields: {
+        foo: { type: GraphQLFloat },
+        bar: { type: myEnum },
+      }
+    });
+
+    expect(astFromValue(
+      { foo: null },
+      inputObj
+    )).to.deep.equal(
+      { kind: 'ObjectValue',
+        fields: [
+          { kind: 'ObjectField',
+            name: { kind: 'Name', value: 'foo' },
+            value: { kind: 'NullValue' } } ] }
+    );
+  });
+ */
+    }
+
     private $complexValue;
 
     private function complexValue()
diff --git a/tests/Validator/ArgumentsOfCorrectTypeTest.php b/tests/Validator/ArgumentsOfCorrectTypeTest.php
index e180d71..ed2175f 100644
--- a/tests/Validator/ArgumentsOfCorrectTypeTest.php
+++ b/tests/Validator/ArgumentsOfCorrectTypeTest.php
@@ -132,6 +132,28 @@ class ArgumentsOfCorrectTypeTest extends TestCase
         ');
     }
 
+    /**
+     * @it null into nullable type
+     */
+    public function testNullIntoNullableType()
+    {
+        $this->expectPassesRule(new ArgumentsOfCorrectType(), '
+        {
+          complicatedArgs {
+            intArgField(intArg: null)
+          }
+        }
+        ');
+
+        $this->expectPassesRule(new ArgumentsOfCorrectType(), '
+        {
+          dog(a: null, b: null, c:{ requiredField: true, intField: null }) {
+            name
+          }
+        }
+        ');
+    }
+
     // Invalid String values
 
     /**
@@ -574,6 +596,20 @@ class ArgumentsOfCorrectTypeTest extends TestCase
         ');
     }
 
+    /**
+     * @it Null value
+     */
+    public function testNullValue()
+    {
+        $this->expectPassesRule(new ArgumentsOfCorrectType(), '
+        {
+          complicatedArgs {
+            stringListArgField(stringListArg: null)
+          }
+        }
+        ');
+    }
+
     /**
      * @it Single value into List
      */
@@ -801,6 +837,24 @@ class ArgumentsOfCorrectTypeTest extends TestCase
         ]);
     }
 
+    /**
+     * @it Null value
+     */
+    public function testNullValue2()
+    {
+        $this->expectFailsRule(new ArgumentsOfCorrectType(), '
+        {
+          complicatedArgs {
+            multipleReqs(req1: null)
+          }
+        }
+        ', [
+            $this->badValue('req1', 'Int!', 'null', 4, 32, [
+                'Expected "Int!", found null.'
+            ]),
+        ]);
+    }
+
 
     // Valid input object value
 
diff --git a/tests/Validator/DefaultValuesOfCorrectTypeTest.php b/tests/Validator/DefaultValuesOfCorrectTypeTest.php
index 21aba50..3dbbe59 100644
--- a/tests/Validator/DefaultValuesOfCorrectTypeTest.php
+++ b/tests/Validator/DefaultValuesOfCorrectTypeTest.php
@@ -50,6 +50,52 @@ class DefaultValuesOfCorrectTypeTest extends TestCase
         ');
     }
 
+    /**
+     * @it variables with valid default null values
+     */
+    public function testVariablesWithValidDefaultNullValues()
+    {
+        $this->expectPassesRule(new DefaultValuesOfCorrectType(), '
+      query WithDefaultValues(
+        $a: Int = null,
+        $b: String = null,
+        $c: ComplexInput = { requiredField: true, intField: null }
+      ) {
+        dog { name }
+      }
+        ');
+    }
+
+    /**
+     * @it variables with invalid default null values
+     */
+    public function testVariablesWithInvalidDefaultNullValues()
+    {
+        $this->expectFailsRule(new DefaultValuesOfCorrectType(), '
+      query WithDefaultValues(
+        $a: Int! = null,
+        $b: String! = null,
+        $c: ComplexInput = { requiredField: null, intField: null }
+      ) {
+        dog { name }
+      }
+        ', [
+            $this->defaultForNonNullArg('a', 'Int!', 'Int', 3, 20),
+            $this->badValue('a', 'Int!', 'null', 3, 20, [
+                'Expected "Int!", found null.'
+            ]),
+            $this->defaultForNonNullArg('b', 'String!', 'String', 4, 23),
+            $this->badValue('b', 'String!', 'null', 4, 23, [
+                'Expected "String!", found null.'
+            ]),
+            $this->badValue('c', 'ComplexInput', '{requiredField: null, intField: null}',
+                5, 28, [
+                    'In field "requiredField": Expected "Boolean!", found null.'
+                ]
+            ),
+        ]);
+    }
+
     /**
      * @it no required variables with default values
      */