--- a/mercurial/commands.py	Fri Aug 26 12:48:20 2016 +0200
+++ b/mercurial/commands.py	Tue Sep 06 09:43:25 2016 -0700
@@ -1411,7 +1411,7 @@
             raise error.Abort(_("--base is incompatible with specifying "
                                "a destination"))
         common = [repo.lookup(rev) for rev in base]
-        heads = revs and map(repo.lookup, revs) or revs
+        heads = revs and map(repo.lookup, revs) or None
         outgoing = discovery.outgoing(repo, common, heads)
         cg = changegroup.getchangegroup(repo, 'bundle', outgoing,
                                         bundlecaps=bundlecaps,