diff -r 2b8373bda032 -r 9cd6578750b9 mercurial/commands.py
--- a/mercurial/commands.py	Fri Aug 24 00:56:56 2007 +0200
+++ b/mercurial/commands.py	Sat Aug 25 11:30:11 2007 +0200
@@ -1928,10 +1928,13 @@
             raise util.Abort(_('repo has %d heads - '
                                'please merge with an explicit rev') %
                              len(heads))
+        parent = repo.dirstate.parents()[0]
         if len(heads) == 1:
-            raise util.Abort(_('there is nothing to merge - '
-                               'use "hg update" instead'))
-        parent = repo.dirstate.parents()[0]
+            msg = _('there is nothing to merge')
+            if parent != repo.lookup(repo.workingctx().branch()):
+                msg = _('%s - use "hg update" instead' % msg)
+            raise util.Abort(msg)
+
         if parent not in heads:
             raise util.Abort(_('working dir not at a head rev - '
                                'use "hg update" or merge with an explicit rev'))