diff -r b32a30da608d -r 45b5eb2941d0 mercurial/commands.py
--- a/mercurial/commands.py	Tue Aug 21 20:51:16 2012 +0200
+++ b/mercurial/commands.py	Wed Aug 22 11:18:35 2012 -0400
@@ -4269,7 +4269,7 @@
                              hint=_("run 'hg heads .' to see heads"))
 
         parent = repo.dirstate.p1()
-        if len(nbhs) == 1:
+        if len(nbhs) <= 1:
             if len(bheads) > 1:
                 raise util.Abort(_("heads are bookmarked - "
                                    "please merge with an explicit rev"),