--- a/mercurial/exchange.py	Thu Jul 24 14:29:08 2014 -0700
+++ b/mercurial/exchange.py	Wed Jul 02 16:13:48 2014 +0200
@@ -242,7 +242,8 @@
     replyhandlers = []
     for partgen in bundle2partsgenerators:
         ret = partgen(pushop, bundler)
-        replyhandlers.append(ret)
+        if callable(ret):
+            replyhandlers.append(ret)
     # do not push if nothing to push
     if bundler.nbparts <= 1:
         return