diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index 73c48d4..1420279 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -224,7 +224,7 @@
       if credential and hasattr(credential, 'set_store'):
         credential.set_store(self)
         if self._cache:
-          self._cache.set(self._key_name, credentials.to_json())
+          self._cache.set(self._key_name, credential.to_json())
 
     return credential