diff --git a/docs/reference.md b/docs/reference.md
index 7ee6f21..70f5614 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -2115,7 +2115,7 @@ static function valueFromASTUntyped($valueNode, array $variables = null)
  * @param Schema $schema
  * @param NamedTypeNode|ListTypeNode|NonNullTypeNode $inputTypeNode
  * @return Type
- * @throws InvariantViolation
+ * @throws \Exception
  */
 static function typeFromAST(GraphQL\Type\Schema $schema, $inputTypeNode)
 ```
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index a92be48..33bcb67 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -12,6 +12,20 @@ use GraphQL\Utils\Utils;
  */
 class InterfaceType extends Type implements AbstractType, OutputType, CompositeType
 {
+    /**
+     * @param mixed $type
+     * @return self
+     */
+    public static function assertInterfaceType($type)
+    {
+        Utils::invariant(
+            $type instanceof self,
+            'Expected ' . Utils::printSafe($type) . ' to be a GraphQL Interface type.'
+        );
+
+        return $type;
+    }
+
     /**
      * @var FieldDefinition[]
      */
diff --git a/src/Type/Definition/ListOfType.php b/src/Type/Definition/ListOfType.php
index 6c45466..5d61d1e 100644
--- a/src/Type/Definition/ListOfType.php
+++ b/src/Type/Definition/ListOfType.php
@@ -20,12 +20,7 @@ class ListOfType extends Type implements WrappingType, OutputType, InputType
      */
     public function __construct($type)
     {
-        if (!$type instanceof Type && !is_callable($type)) {
-            throw new InvariantViolation(
-                'Can only create List of a GraphQLType but got: ' . Utils::printSafe($type)
-            );
-        }
-        $this->ofType = $type;
+        $this->ofType = Type::assertType($type);
     }
 
     /**
diff --git a/src/Type/Definition/NonNull.php b/src/Type/Definition/NonNull.php
index 4499c74..2f3912d 100644
--- a/src/Type/Definition/NonNull.php
+++ b/src/Type/Definition/NonNull.php
@@ -11,7 +11,35 @@ use GraphQL\Utils\Utils;
 class NonNull extends Type implements WrappingType, OutputType, InputType
 {
     /**
-     * @var ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType
+     * @param mixed $type
+     * @return self
+     */
+    public static function assertNullType($type)
+    {
+        Utils::invariant(
+            $type instanceof self,
+            'Expected ' . Utils::printSafe($type) . ' to be a GraphQL Non-Null type.'
+        );
+
+        return $type;
+    }
+
+    /**
+     * @param mixed $type
+     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
+     */
+    public static function assertNullableType($type)
+    {
+        Utils::invariant(
+            Type::isType($type) && !$type instanceof self,
+            'Expected ' . Utils::printSafe($type) . ' to be a GraphQL nullable type.'
+        );
+
+        return $type;
+    }
+
+    /**
+     * @var ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
      */
     private $ofType;
 
@@ -21,37 +49,17 @@ class NonNull extends Type implements WrappingType, OutputType, InputType
      */
     public function __construct($type)
     {
-        if (!$type instanceof Type && !is_callable($type)) {
-            throw new InvariantViolation(
-                'Can only create NonNull of a Nullable GraphQLType but got: ' . Utils::printSafe($type)
-            );
-        }
-        if ($type instanceof NonNull) {
-            throw new InvariantViolation(
-                'Can only create NonNull of a Nullable GraphQLType but got: ' . Utils::printSafe($type)
-            );
-        }
-        Utils::invariant(
-            !($type instanceof NonNull),
-            'Cannot nest NonNull inside NonNull'
-        );
-        $this->ofType = $type;
+        $this->ofType = self::assertNullableType($type);
     }
 
     /**
      * @param bool $recurse
-     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType
+     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
      * @throws InvariantViolation
      */
     public function getWrappedType($recurse = false)
     {
         $type = $this->ofType;
-
-        Utils::invariant(
-            !($type instanceof NonNull),
-            'Cannot nest NonNull inside NonNull'
-        );
-
         return ($recurse && $type instanceof WrappingType) ? $type->getWrappedType($recurse) : $type;
     }
 
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index 5f2d3e1..5c5d0eb 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -49,6 +49,20 @@ use GraphQL\Utils\Utils;
  */
 class ObjectType extends Type implements OutputType, CompositeType
 {
+    /**
+     * @param mixed $type
+     * @return self
+     */
+    public static function assertObjectType($type)
+    {
+        Utils::invariant(
+            $type instanceof self,
+            'Expected ' . Utils::printSafe($type) . ' to be a GraphQL Object type.'
+        );
+
+        return $type;
+    }
+
     /**
      * @var FieldDefinition[]
      */
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index 5afceb8..ba797bc 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -4,8 +4,10 @@ namespace GraphQL\Type\Definition;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\ListType;
 use GraphQL\Language\AST\NamedType;
+use GraphQL\Language\AST\NonNullType;
 use GraphQL\Language\AST\TypeDefinitionNode;
 use GraphQL\Type\Introspection;
+use GraphQL\Utils\Utils;
 
 /**
  * Registry of standard GraphQL types
@@ -218,11 +220,25 @@ abstract class Type implements \JsonSerializable
             $type instanceof UnionType ||
             $type instanceof EnumType ||
             $type instanceof InputObjectType ||
-            $type instanceof ListType ||
+            $type instanceof ListOfType ||
             $type instanceof NonNull
         );
     }
 
+    /**
+     * @param mixed $type
+     * @return mixed
+     */
+    public static function assertType($type)
+    {
+        Utils::invariant(
+            self::isType($type),
+            'Expected ' . Utils::printSafe($type) . ' to be a GraphQL type.'
+        );
+
+        return $type;
+    }
+
     /**
      * @api
      * @param Type $type
diff --git a/src/Utils/AST.php b/src/Utils/AST.php
index d5c867e..9a18915 100644
--- a/src/Utils/AST.php
+++ b/src/Utils/AST.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Utils;
 
+use GraphQL\Error\Error;
 use GraphQL\Error\InvariantViolation;
 use GraphQL\Language\AST\BooleanValueNode;
 use GraphQL\Language\AST\DocumentNode;
@@ -205,51 +206,50 @@ class AST
             return new ObjectValueNode(['fields' => $fieldNodes]);
         }
 
-        Utils::invariant(
-            $type instanceof ScalarType || $type instanceof EnumType,
-            "Must provide Input Type, cannot use: " . Utils::printSafe($type)
-        );
+        if ($type instanceof ScalarType || $type instanceof EnumType) {
+            // Since value is an internally represented value, it must be serialized
+            // to an externally represented value before converting into an AST.
+            $serialized = $type->serialize($value);
+            if (null === $serialized || Utils::isInvalid($serialized)) {
+                return null;
+            }
 
-        // Since value is an internally represented value, it must be serialized
-        // to an externally represented value before converting into an AST.
-        $serialized = $type->serialize($value);
-        if (null === $serialized || Utils::isInvalid($serialized)) {
-            return null;
-        }
-
-        // Others serialize based on their corresponding PHP scalar types.
-        if (is_bool($serialized)) {
-            return new BooleanValueNode(['value' => $serialized]);
-        }
-        if (is_int($serialized)) {
-            return new IntValueNode(['value' => $serialized]);
-        }
-        if (is_float($serialized)) {
-            if ((int) $serialized == $serialized) {
+            // Others serialize based on their corresponding PHP scalar types.
+            if (is_bool($serialized)) {
+                return new BooleanValueNode(['value' => $serialized]);
+            }
+            if (is_int($serialized)) {
                 return new IntValueNode(['value' => $serialized]);
             }
-            return new FloatValueNode(['value' => $serialized]);
-        }
-        if (is_string($serialized)) {
-            // Enum types use Enum literals.
-            if ($type instanceof EnumType) {
-                return new EnumValueNode(['value' => $serialized]);
+            if (is_float($serialized)) {
+                if ((int) $serialized == $serialized) {
+                    return new IntValueNode(['value' => $serialized]);
+                }
+                return new FloatValueNode(['value' => $serialized]);
+            }
+            if (is_string($serialized)) {
+                // Enum types use Enum literals.
+                if ($type instanceof EnumType) {
+                    return new EnumValueNode(['value' => $serialized]);
+                }
+
+                // ID types can use Int literals.
+                $asInt = (int) $serialized;
+                if ($type instanceof IDType && (string) $asInt === $serialized) {
+                    return new IntValueNode(['value' => $serialized]);
+                }
+
+                // Use json_encode, which uses the same string encoding as GraphQL,
+                // then remove the quotes.
+                return new StringValueNode([
+                    'value' => substr(json_encode($serialized), 1, -1)
+                ]);
             }
 
-            // ID types can use Int literals.
-            $asInt = (int) $serialized;
-            if ($type instanceof IDType && (string) $asInt === $serialized) {
-                return new IntValueNode(['value' => $serialized]);
-            }
-
-            // Use json_encode, which uses the same string encoding as GraphQL,
-            // then remove the quotes.
-            return new StringValueNode([
-                'value' => substr(json_encode($serialized), 1, -1)
-            ]);
+            throw new InvariantViolation('Cannot convert value to AST: ' . Utils::printSafe($serialized));
         }
 
-        throw new InvariantViolation('Cannot convert value to AST: ' . Utils::printSafe($serialized));
+        throw new Error('Unknown type: ' . Utils::printSafe($type) . '.');
     }
 
     /**
@@ -395,25 +395,26 @@ class AST
             return $enumValue->value;
         }
 
-        Utils::invariant($type instanceof ScalarType, 'Must be scalar type');
-        /** @var ScalarType $type */
+        if ($type instanceof ScalarType) {
+            // Scalars fulfill parsing a literal value via parseLiteral().
+            // Invalid values represent a failure to parse correctly, in which case
+            // no value is returned.
+            try {
+                $result = $type->parseLiteral($valueNode, $variables);
+            } catch (\Exception $error) {
+                return $undefined;
+            } catch (\Throwable $error) {
+                return $undefined;
+            }
 
-        // Scalars fulfill parsing a literal value via parseLiteral().
-        // Invalid values represent a failure to parse correctly, in which case
-        // no value is returned.
-        try {
-            $result = $type->parseLiteral($valueNode, $variables);
-        } catch (\Exception $error) {
-            return $undefined;
-        } catch (\Throwable $error) {
-            return $undefined;
+            if (Utils::isInvalid($result)) {
+                return $undefined;
+            }
+
+            return $result;
         }
 
-        if (Utils::isInvalid($result)) {
-            return $undefined;
-        }
-
-        return $result;
+        throw new Error('Unknown type: ' . Utils::printSafe($type) . '.');
     }
 
     /**
@@ -473,9 +474,9 @@ class AST
                 return ($variables && isset($variables[$variableName]) && !Utils::isInvalid($variables[$variableName]))
                     ? $variables[$variableName]
                     : null;
-            default:
-                throw new InvariantViolation('Unexpected value kind: ' . $valueNode->kind);
-          }
+        }
+
+        throw new Error('Unexpected value kind: ' . $valueNode->kind . '.');
     }
 
     /**
@@ -485,7 +486,7 @@ class AST
      * @param Schema $schema
      * @param NamedTypeNode|ListTypeNode|NonNullTypeNode $inputTypeNode
      * @return Type
-     * @throws InvariantViolation
+     * @throws \Exception
      */
     public static function typeFromAST(Schema $schema, $inputTypeNode)
     {
@@ -497,9 +498,11 @@ class AST
             $innerType = self::typeFromAST($schema, $inputTypeNode->type);
             return $innerType ? new NonNull($innerType) : null;
         }
+        if ($inputTypeNode instanceof NamedTypeNode) {
+            return $schema->getType($inputTypeNode->name->value);
+        }
 
-        Utils::invariant($inputTypeNode && $inputTypeNode instanceof NamedTypeNode, 'Must be a named type');
-        return $schema->getType($inputTypeNode->name->value);
+        throw new Error('Unexpected type kind: ' . $inputTypeNode->kind . '.');
     }
 
     /**
diff --git a/src/Utils/ASTDefinitionBuilder.php b/src/Utils/ASTDefinitionBuilder.php
index 073e773..c5516dd 100644
--- a/src/Utils/ASTDefinitionBuilder.php
+++ b/src/Utils/ASTDefinitionBuilder.php
@@ -75,8 +75,7 @@ class ASTDefinitionBuilder
         }
         if ($inputTypeNode->kind == NodeKind::NON_NULL_TYPE) {
             $wrappedType = $this->buildWrappedType($innerType, $inputTypeNode->type);
-            Utils::invariant(!($wrappedType instanceof NonNull), 'No nesting nonnull.');
-            return Type::nonNull($wrappedType);
+            return Type::nonNull(NonNull::assertNullableType($wrappedType));
         }
         return $innerType;
     }
@@ -159,8 +158,7 @@ class ASTDefinitionBuilder
     public function buildObjectType($typeNode)
     {
         $type = $this->buildType($typeNode);
-        Utils::invariant($type instanceof ObjectType, 'Expected Object type.' . get_class($type));
-        return $type;
+        return ObjectType::assertObjectType($type);
     }
 
     /**
@@ -171,8 +169,7 @@ class ASTDefinitionBuilder
     public function buildInterfaceType($typeNode)
     {
         $type = $this->buildType($typeNode);
-        Utils::invariant($type instanceof InterfaceType, 'Expected Interface type.');
-        return $type;
+        return InterfaceType::assertInterfaceType($type);
     }
 
     /**
diff --git a/src/Utils/FindBreakingChanges.php b/src/Utils/FindBreakingChanges.php
index 63acbef..6c67aa1 100644
--- a/src/Utils/FindBreakingChanges.php
+++ b/src/Utils/FindBreakingChanges.php
@@ -148,8 +148,11 @@ class FindBreakingChanges
         $dangerousChanges = [];
         foreach ($oldTypeMap as $oldTypeName => $oldTypeDefinition) {
             $newTypeDefinition = isset($newTypeMap[$oldTypeName]) ? $newTypeMap[$oldTypeName] : null;
-            if (!($oldTypeDefinition instanceof ObjectType || $oldTypeDefinition instanceof InterfaceType) ||
-                !($newTypeDefinition instanceof $oldTypeDefinition)) {
+            if (
+                !($oldTypeDefinition instanceof ObjectType || $oldTypeDefinition instanceof InterfaceType) ||
+                !($newTypeDefinition instanceof ObjectType || $newTypeDefinition instanceof InterfaceType) ||
+                !($newTypeDefinition instanceof $oldTypeDefinition)
+            ) {
                 continue;
             }
 
@@ -262,7 +265,11 @@ class FindBreakingChanges
         $breakingChanges = [];
         foreach ($oldTypeMap as $typeName => $oldType) {
             $newType = isset($newTypeMap[$typeName]) ? $newTypeMap[$typeName] : null;
-            if (!($oldType instanceof ObjectType || $oldType instanceof InterfaceType) || !($newType instanceof $oldType)) {
+            if (
+                !($oldType instanceof ObjectType || $oldType instanceof InterfaceType) ||
+                !($newType instanceof ObjectType || $newType instanceof InterfaceType) ||
+                !($newType instanceof $oldType)
+            ) {
                 continue;
             }
             $oldTypeFieldsDef = $oldType->getFields();
diff --git a/src/Utils/SchemaPrinter.php b/src/Utils/SchemaPrinter.php
index d80e104..e0a74b7 100644
--- a/src/Utils/SchemaPrinter.php
+++ b/src/Utils/SchemaPrinter.php
@@ -1,6 +1,7 @@
 <?php
 namespace GraphQL\Utils;
 
+use GraphQL\Error\Error;
 use GraphQL\Language\Printer;
 use GraphQL\Type\Introspection;
 use GraphQL\Type\Schema;
@@ -142,9 +143,11 @@ class SchemaPrinter
             return self::printUnion($type, $options);
         } else if ($type instanceof EnumType) {
             return self::printEnum($type, $options);
+        } else if ($type instanceof InputObjectType) {
+            return self::printInputObject($type, $options);
         }
-        Utils::invariant($type instanceof InputObjectType);
-        return self::printInputObject($type, $options);
+
+        throw new Error('Unknown type: ' . Utils::printSafe($type) . '.');
     }
 
     private static function printScalar(ScalarType $type, array $options)
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index cf7f30c..1a454f7 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -319,27 +319,28 @@ class DocumentValidator
             return [];
         }
 
-        Utils::invariant($type instanceof ScalarType, 'Must be a scalar type');
-        /** @var ScalarType $type */
-
-        // Scalars determine if a literal values is valid via parseLiteral().
-        try {
-            $parseResult = $type->parseLiteral($valueNode);
-            if (Utils::isInvalid($parseResult)) {
+        if ($type instanceof ScalarType) {
+            // Scalars determine if a literal values is valid via parseLiteral().
+            try {
+                $parseResult = $type->parseLiteral($valueNode);
+                if (Utils::isInvalid($parseResult)) {
+                    $printed = Printer::doPrint($valueNode);
+                    return ["Expected type \"{$type->name}\", found $printed."];
+                }
+            } catch (\Exception $error) {
                 $printed = Printer::doPrint($valueNode);
-                return ["Expected type \"{$type->name}\", found $printed."];
+                $message = $error->getMessage();
+                return ["Expected type \"{$type->name}\", found $printed; $message"];
+            } catch (\Throwable $error) {
+                $printed = Printer::doPrint($valueNode);
+                $message = $error->getMessage();
+                return ["Expected type \"{$type->name}\", found $printed; $message"];
             }
-        } catch (\Exception $error) {
-            $printed = Printer::doPrint($valueNode);
-            $message = $error->getMessage();
-            return ["Expected type \"{$type->name}\", found $printed; $message"];
-        } catch (\Throwable $error) {
-            $printed = Printer::doPrint($valueNode);
-            $message = $error->getMessage();
-            return ["Expected type \"{$type->name}\", found $printed; $message"];
+
+            return [];
         }
 
-        return [];
+        throw new Error('Unknown type: ' . Utils::printSafe($type) . '.');
     }
 
     /**
diff --git a/tests/Type/DefinitionTest.php b/tests/Type/DefinitionTest.php
index 1622220..9f1b8ce 100644
--- a/tests/Type/DefinitionTest.php
+++ b/tests/Type/DefinitionTest.php
@@ -468,15 +468,6 @@ class DefinitionTest extends \PHPUnit_Framework_TestCase
         }
     }
 
-    /**
-     * @it prohibits nesting NonNull inside NonNull
-     */
-    public function testProhibitsNonNullNesting()
-    {
-        $this->setExpectedException('\Exception');
-        new NonNull(new NonNull(Type::int()));
-    }
-
     /**
      * @it prohibits putting non-Object types in unions
      */
diff --git a/tests/Type/ValidationTest.php b/tests/Type/ValidationTest.php
index 373180d..1409da4 100644
--- a/tests/Type/ValidationTest.php
+++ b/tests/Type/ValidationTest.php
@@ -1975,7 +1975,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
     }
 
 
-    // DESCRIBE: Type System: List must accept GraphQL types
+    // DESCRIBE: Type System: List must accept only types
 
     /**
      * @it accepts an type as item type of list
@@ -2022,7 +2022,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
                 $this->fail("Expected exception not thrown for: " . Utils::printSafe($type));
             } catch (InvariantViolation $e) {
                 $this->assertEquals(
-                    'Can only create List of a GraphQLType but got: ' . Utils::printSafe($type),
+                    'Expected '. Utils::printSafe($type) . ' to be a GraphQL type.',
                     $e->getMessage()
                 );
             }
@@ -2030,7 +2030,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
     }
 
 
-    // DESCRIBE: Type System: NonNull must accept GraphQL types
+    // DESCRIBE: Type System: NonNull must only accept non-nullable types
 
     /**
      * @it accepts an type as nullable type of non-null
@@ -2057,8 +2057,6 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
         }
     }
 
-    // TODO: rejects a non-type as nullable type of non-null: ${type}
-
     /**
      * @it rejects a non-type as nullable type of non-null
      */
@@ -2079,7 +2077,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
                 $this->fail("Expected exception not thrown for: " . Utils::printSafe($type));
             } catch (InvariantViolation $e) {
                 $this->assertEquals(
-                    'Can only create NonNull of a Nullable GraphQLType but got: ' . Utils::printSafe($type),
+                    'Expected ' . Utils::printSafe($type) . ' to be a GraphQL nullable type.',
                     $e->getMessage()
                 );
             }