diff --git a/txsyncml/resource.py b/txsyncml/resource.py
index 14c51a8..8d700d6 100644
--- a/txsyncml/resource.py
+++ b/txsyncml/resource.py
@@ -110,4 +110,4 @@ def handle_authorized_syncml(self, user_state, doc):
         return SyncML.create(header=header, body=body)
 
     def handle_unauthorized_syncml(self, failure):
-        return ''
+        raise NotImplementedError('Not implemented yet')