diff --git a/django_walletpass/classviews.py b/django_walletpass/classviews.py
index da2872b..ffecb2a 100644
--- a/django_walletpass/classviews.py
+++ b/django_walletpass/classviews.py
@@ -113,7 +113,11 @@ def retrieve(self, request, pass_type_id, serial_number):
         response['Content-Disposition'] = 'attachment; filename=pass.pkpass'
 
         response['Last-Modified'] = http_date(timegm(pass_.updated_at.utctimetuple()))
-        return ConditionalGetMiddleware(get_response=response)(request)
+
+        def _get_response(request):
+             return response
+
+        return ConditionalGetMiddleware(get_response=_get_response)(request)
 
 
 # TODO: use ModelViewSet