diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index 8a24fce..73c48d4 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -409,7 +409,7 @@
     user = users.get_current_user()
     memcache.set(user.user_id(), pickle.dumps(self.flow),
                  namespace=OAUTH2CLIENT_NAMESPACE)
-    return url
+    return str(url)
 
   def http(self):
     """Returns an authorized http instance.