diff -r 2a8b6d78d7ee -r 0d51eb296fb9 mercurial/merge.py
--- a/mercurial/merge.py	Thu Mar 15 18:57:47 2007 -0500
+++ b/mercurial/merge.py	Fri Mar 16 00:45:18 2007 -0300
@@ -450,7 +450,10 @@
     wc = repo.workingctx()
     if node is None:
         # tip of current branch
-        node = repo.branchtags()[wc.branch()]
+        try:
+            node = repo.branchtags()[wc.branch()]
+        except KeyError:
+            raise util.Abort(_("branch %s not found") % wc.branch())
     overwrite = force and not branchmerge
     forcemerge = force and branchmerge
     pl = wc.parents()