diff --git a/graphene/types/objecttype.py b/graphene/types/objecttype.py
index f4a0f5a0..c4d969b4 100644
--- a/graphene/types/objecttype.py
+++ b/graphene/types/objecttype.py
@@ -1,3 +1,5 @@
+import inspect
+
 from .base import BaseOptions, BaseType, BaseTypeMeta
 from .field import Field
 from .interface import Interface
@@ -137,7 +139,7 @@ class ObjectType(BaseType, metaclass=ObjectTypeMeta):
         fields = {}
 
         for interface in interfaces:
-            assert issubclass(
+            assert inspect.isclass(interface) and issubclass(
                 interface, Interface
             ), f'All interfaces of {cls.__name__} must be a subclass of Interface. Received "{interface}".'
             fields.update(interface._meta.fields)
diff --git a/graphene/types/schema.py b/graphene/types/schema.py
index 2d1eaf6b..f40975b6 100644
--- a/graphene/types/schema.py
+++ b/graphene/types/schema.py
@@ -258,7 +258,8 @@ class TypeMap(dict):
             union_types = []
             for graphene_objecttype in graphene_type._meta.types:
                 object_type = create_graphql_type(graphene_objecttype)
-                assert object_type.graphene_type == graphene_objecttype
+                if hasattr(object_type, "graphene_type"):
+                    assert object_type.graphene_type == graphene_objecttype
                 union_types.append(object_type)
             return union_types
 
diff --git a/graphene/types/tests/test_type_map.py b/graphene/types/tests/test_type_map.py
index 41327211..25974ca4 100644
--- a/graphene/types/tests/test_type_map.py
+++ b/graphene/types/tests/test_type_map.py
@@ -1,5 +1,6 @@
 from textwrap import dedent
 
+import pytest
 from graphql.type import (
     GraphQLArgument,
     GraphQLEnumType,
@@ -22,6 +23,7 @@ from ..objecttype import ObjectType
 from ..scalars import Int, String
 from ..structures import List, NonNull
 from ..schema import Schema
+from ..union import Union
 
 
 def create_type_map(types, auto_camelcase=True):
@@ -313,3 +315,80 @@ def test_graphql_type():
     )
     assert not results.errors
     assert results.data == {"graphqlType": {"hello": "world"}}
+
+
+def test_graphql_type_interface():
+    MyGraphQLInterface = GraphQLInterfaceType(
+        name="MyGraphQLType",
+        fields={
+            "hello": GraphQLField(GraphQLString, resolve=lambda obj, info: "world")
+        },
+    )
+
+    with pytest.raises(AssertionError) as error:
+
+        class MyGrapheneType(ObjectType):
+            class Meta:
+                interfaces = (MyGraphQLInterface,)
+
+    assert str(error.value) == (
+        "All interfaces of MyGrapheneType must be a subclass of Interface. "
+        'Received "MyGraphQLType".'
+    )
+
+
+def test_graphql_type_union():
+    MyGraphQLType = GraphQLObjectType(
+        name="MyGraphQLType",
+        fields={
+            "hello": GraphQLField(GraphQLString, resolve=lambda obj, info: "world")
+        },
+    )
+
+    class MyGrapheneType(ObjectType):
+        hi = String(default_value="world")
+
+    class MyUnion(Union):
+        class Meta:
+            types = (MyGraphQLType, MyGrapheneType)
+
+        @classmethod
+        def resolve_type(cls, instance, info):
+            return MyGraphQLType
+
+    class Query(ObjectType):
+        my_union = Field(MyUnion)
+
+        def resolve_my_union(root, info):
+            return {}
+
+    schema = Schema(query=Query)
+    assert str(schema) == dedent(
+        """\
+        type Query {
+          myUnion: MyUnion
+        }
+
+        union MyUnion = MyGraphQLType | MyGrapheneType
+
+        type MyGraphQLType {
+          hello: String
+        }
+
+        type MyGrapheneType {
+          hi: String
+        }
+    """
+    )
+
+    results = schema.execute(
+        """
+        query {
+            myUnion {
+                __typename
+            }
+        }
+    """
+    )
+    assert not results.errors
+    assert results.data == {"myUnion": {"__typename": "MyGraphQLType"}}