diff --git a/gluon/tools.py b/gluon/tools.py
index f81d7eb4..d844cee8 100644
--- a/gluon/tools.py
+++ b/gluon/tools.py
@@ -4416,7 +4416,7 @@ class Service(object):
             if must_respond:
                 return return_response(id, s)
             else:
-                return
+                return ''
         except Service.JsonRpcException, e:
             return return_error(id, e.code, e.info)
         except BaseException: