Fix tests

graphene/types/definitions.py

@@ -7,6 +7,7 @@ from graphql import (
     GraphQLObjectType,
     GraphQLScalarType,
     GraphQLUnionType,
+    Undefined,
 )
 
 
@@ -49,7 +50,7 @@ class GrapheneEnumType(GrapheneGraphQLType, GraphQLEnumType):
                 try:
                     value = enum[value]
                 except KeyError:
-                    value = None
+                    return Undefined
         return super(GrapheneEnumType, self).serialize(value)
 
 

graphene/types/tests/test_enum.py

@@ -234,8 +234,10 @@ def test_enum_types():
     from enum import Enum as PyEnum
 
     class Color(PyEnum):
+        """Primary colors"""
+
         RED = 1
-        GREEN = 2
+        YELLOW = 2
         BLUE = 3
 
     GColor = Enum.from_enum(Color)
@@ -250,16 +252,16 @@ def test_enum_types():
 
     assert str(schema) == dedent(
         '''\
-        """An enumeration."""
-        enum Color {
-          RED
-          GREEN
-          BLUE
-        }
-
         type Query {
           color: Color!
         }
+
+        """Primary colors"""
+        enum Color {
+          RED
+          YELLOW
+          BLUE
+        }
     '''
     )