Index: django/core/handlers/wsgi.py
===================================================================
--- django/core/handlers/wsgi.py	(Revision 7409)
+++ django/core/handlers/wsgi.py	(Arbeitskopie)
@@ -207,6 +207,8 @@
                 # Apply response middleware
                 for middleware_method in self._response_middleware:
                     response = middleware_method(request, response)
+                    assert not response is None, 'Response middleware method returned None %s' % (
+                        middleware_method)
                 response = self.apply_response_fixes(request, response)
         finally:
             dispatcher.send(signal=signals.request_finished)