diff --git a/graphene/tests/__init__.py b/graphene/tests/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/graphene/tests/issues/__init__.py b/graphene/tests/issues/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/graphene/tests/issues/test_313.py b/graphene/tests/issues/test_313.py
new file mode 100644
index 00000000..bed87290
--- /dev/null
+++ b/graphene/tests/issues/test_313.py
@@ -0,0 +1,52 @@
+# https://github.com/graphql-python/graphene/issues/313
+
+import graphene
+from graphene import resolve_only_args
+
+class Success(graphene.ObjectType):
+    yeah = graphene.String()
+
+
+class Error(graphene.ObjectType):
+    message = graphene.String()
+
+
+class CreatePostResult(graphene.Union):
+    class Meta:
+        types = [Success, Error]
+
+
+class CreatePost(graphene.Mutation):
+    class Input:
+        text = graphene.String(required=True)
+
+    result = graphene.Field(CreatePostResult)
+
+    @resolve_only_args
+    def mutate(self, text):
+        result = Success(yeah='yeah')
+
+        return CreatePost(result=result)
+
+
+class Mutations(graphene.ObjectType):
+    create_post = CreatePost.Field()
+
+# tests.py
+
+def test_create_post():
+    query_string = '''
+    mutation {
+      createPost(text: "Try this out") {
+        result {
+          __typename
+        }
+      }
+    }
+    '''
+
+    schema = graphene.Schema(mutation=Mutations)
+    result = schema.execute(query_string)
+
+    assert not result.errors
+    assert result.data['createPost']['result']['__typename'] == 'Success'
\ No newline at end of file
diff --git a/graphene/types/union.py b/graphene/types/union.py
index 622f465e..3d236000 100644
--- a/graphene/types/union.py
+++ b/graphene/types/union.py
@@ -39,7 +39,11 @@ class Union(six.with_metaclass(UnionMeta)):
     to determine which type is actually used when the field is resolved.
     '''
 
-    resolve_type = None
+    @classmethod
+    def resolve_type(cls, instance, context, info):
+        from .objecttype import ObjectType
+        if isinstance(instance, ObjectType):
+            return type(instance)
 
     def __init__(self, *args, **kwargs):
         raise Exception("A Union cannot be intitialized")