diff --git a/src/Type/Definition/EnumType.php b/src/Type/Definition/EnumType.php
index 2b42590..b565a25 100644
--- a/src/Type/Definition/EnumType.php
+++ b/src/Type/Definition/EnumType.php
@@ -22,7 +22,7 @@ use function sprintf;
 /**
  * Class EnumType
  */
-class EnumType extends Type implements InputType, OutputType, LeafType, NamedType
+class EnumType extends Type implements InputType, OutputType, LeafType, NullableType, NamedType
 {
     /** @var EnumTypeDefinitionNode|null */
     public $astNode;
diff --git a/src/Type/Definition/InputObjectType.php b/src/Type/Definition/InputObjectType.php
index e43caad..756e142 100644
--- a/src/Type/Definition/InputObjectType.php
+++ b/src/Type/Definition/InputObjectType.php
@@ -18,7 +18,7 @@ use function sprintf;
 /**
  * Class InputObjectType
  */
-class InputObjectType extends Type implements InputType, NamedType
+class InputObjectType extends Type implements InputType, NullableType, NamedType
 {
     /** @var InputObjectTypeDefinitionNode|null */
     public $astNode;
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index ff01a48..52ad81d 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -15,7 +15,7 @@ use function sprintf;
 /**
  * Class InterfaceType
  */
-class InterfaceType extends Type implements AbstractType, OutputType, CompositeType, NamedType
+class InterfaceType extends Type implements AbstractType, OutputType, CompositeType, NullableType, NamedType
 {
     /** @var InterfaceTypeDefinitionNode|null */
     public $astNode;
diff --git a/src/Type/Definition/ListOfType.php b/src/Type/Definition/ListOfType.php
index 7fdb6e6..29e3438 100644
--- a/src/Type/Definition/ListOfType.php
+++ b/src/Type/Definition/ListOfType.php
@@ -7,7 +7,7 @@ namespace GraphQL\Type\Definition;
 /**
  * Class ListOfType
  */
-class ListOfType extends Type implements WrappingType, OutputType, InputType
+class ListOfType extends Type implements WrappingType, OutputType, NullableType, InputType
 {
     /** @var ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType */
     public $ofType;
diff --git a/src/Type/Definition/NonNull.php b/src/Type/Definition/NonNull.php
index f3ebbca..cc80be0 100644
--- a/src/Type/Definition/NonNull.php
+++ b/src/Type/Definition/NonNull.php
@@ -4,8 +4,6 @@ declare(strict_types=1);
 
 namespace GraphQL\Type\Definition;
 
-use Exception;
-use GraphQL\Error\InvariantViolation;
 use GraphQL\Utils\Utils;
 
 /**
@@ -13,13 +11,11 @@ use GraphQL\Utils\Utils;
  */
 class NonNull extends Type implements WrappingType, OutputType, InputType
 {
-    /** @var ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType */
+    /** @var NullableType */
     private $ofType;
 
     /**
-     * @param callable|Type $type
-     *
-     * @throws Exception
+     * @param NullableType $type
      */
     public function __construct($type)
     {
@@ -29,7 +25,7 @@ class NonNull extends Type implements WrappingType, OutputType, InputType
     /**
      * @param mixed $type
      *
-     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
+     * @return NullableType
      */
     public static function assertNullableType($type)
     {
@@ -67,9 +63,7 @@ class NonNull extends Type implements WrappingType, OutputType, InputType
     /**
      * @param bool $recurse
      *
-     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
-     *
-     * @throws InvariantViolation
+     * @return Type
      */
     public function getWrappedType($recurse = false)
     {
diff --git a/src/Type/Definition/NullableType.php b/src/Type/Definition/NullableType.php
new file mode 100644
index 0000000..7ff70b2
--- /dev/null
+++ b/src/Type/Definition/NullableType.php
@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace GraphQL\Type\Definition;
+
+/*
+export type GraphQLNullableType =
+  | GraphQLScalarType
+  | GraphQLObjectType
+  | GraphQLInterfaceType
+  | GraphQLUnionType
+  | GraphQLEnumType
+  | GraphQLInputObjectType
+  | GraphQLList<any>;
+ */
+
+interface NullableType
+{
+}
diff --git a/src/Type/Definition/ObjectType.php b/src/Type/Definition/ObjectType.php
index ac40258..f509293 100644
--- a/src/Type/Definition/ObjectType.php
+++ b/src/Type/Definition/ObjectType.php
@@ -54,7 +54,7 @@ use function sprintf;
  *        }
  *     ]);
  */
-class ObjectType extends Type implements OutputType, CompositeType, NamedType
+class ObjectType extends Type implements OutputType, CompositeType, NullableType, NamedType
 {
     /** @var ObjectTypeDefinitionNode|null */
     public $astNode;
diff --git a/src/Type/Definition/ScalarType.php b/src/Type/Definition/ScalarType.php
index db4e63f..17bf6e7 100644
--- a/src/Type/Definition/ScalarType.php
+++ b/src/Type/Definition/ScalarType.php
@@ -27,7 +27,7 @@ use function is_string;
  *     }
  * }
  */
-abstract class ScalarType extends Type implements OutputType, InputType, LeafType, NamedType
+abstract class ScalarType extends Type implements OutputType, InputType, LeafType, NullableType, NamedType
 {
     /** @var ScalarTypeDefinitionNode|null */
     public $astNode;
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index c349d11..5b8e414 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -137,7 +137,7 @@ abstract class Type implements JsonSerializable
     }
 
     /**
-     * @param ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType $wrappedType
+     * @param NullableType $wrappedType
      *
      * @return NonNull
      *
@@ -338,7 +338,7 @@ abstract class Type implements JsonSerializable
     /**
      * @param Type $type
      *
-     * @return ObjectType|InterfaceType|UnionType|ScalarType|InputObjectType|EnumType|ListOfType
+     * @return NullableType
      *
      * @api
      */
diff --git a/src/Type/Definition/UnionType.php b/src/Type/Definition/UnionType.php
index a8e6fd7..8a735e3 100644
--- a/src/Type/Definition/UnionType.php
+++ b/src/Type/Definition/UnionType.php
@@ -17,7 +17,7 @@ use function sprintf;
 /**
  * Class UnionType
  */
-class UnionType extends Type implements AbstractType, OutputType, CompositeType, NamedType
+class UnionType extends Type implements AbstractType, OutputType, CompositeType, NullableType, NamedType
 {
     /** @var UnionTypeDefinitionNode */
     public $astNode;