diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index a6dbf5d77..0a2e103f9 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -33,7 +33,7 @@ class DictWithMetadata(dict):
         """
         # return an instance of the first dict in MRO that isn't a DictWithMetadata
         for base in self.__class__.__mro__:
-            if not isinstance(base, DictWithMetadata) and isinstance(base, dict):
+            if not issubclass(base, DictWithMetadata) and issubclass(base, dict):
                 return base(self)
 
 
diff --git a/rest_framework/tests/serializer.py b/rest_framework/tests/serializer.py
index 2d17e99df..193795521 100644
--- a/rest_framework/tests/serializer.py
+++ b/rest_framework/tests/serializer.py
@@ -900,6 +900,13 @@ class SerializerPickleTests(TestCase):
                 fields = ('name', 'age')
         pickle.dumps(InnerPersonSerializer(Person(name="Noah", age=950)).data)
 
+    def test_getstate_method_should_not_return_none(self):
+        '''Regression test for 
+        https://github.com/tomchristie/django-rest-framework/issues/645
+        '''
+        d = serializers.DictWithMetadata({1:1})
+        self.assertEqual(d.__getstate__(), serializers.SortedDict({1:1}))
+
 
 class DepthTest(TestCase):
     def test_implicit_nesting(self):