--- a/mercurial/hg.py	Thu Mar 18 01:58:26 2010 +0100
+++ b/mercurial/hg.py	Thu Mar 18 16:38:07 2010 +0100
@@ -209,6 +209,7 @@
         src_repo = repository(ui, source)
     else:
         src_repo = source
+        branch = None
         origsource = source = src_repo.url()
     rev, checkout = addbranchrevs(src_repo, src_repo, branch, rev)