--- a/mercurial/commands.py	Mon Jun 23 15:03:22 2008 +0200
+++ b/mercurial/commands.py	Mon Jun 23 15:02:52 2008 +0200
@@ -1891,7 +1891,7 @@
     performed before any further updates are allowed.
 
     If no revision is specified, the working directory's parent is a
-    head revision, and the repository contains exactly one other head,
+    head revision, and the current branch contains exactly one other head,
     the other head is merged with by default. Otherwise, an explicit
     revision to merge with must be provided.
     """
@@ -1902,22 +1902,28 @@
         node = rev
 
     if not node:
-        heads = repo.heads()
-        if len(heads) > 2:
-            raise util.Abort(_('repo has %d heads - '
-                               'please merge with an explicit rev') %
-                             len(heads))
+        branch = repo.workingctx().branch()
+        bheads = repo.branchheads()
+        if len(bheads) > 2:
+            raise util.Abort(_("branch '%s' has %d heads - "
+                               "please merge with an explicit rev") %
+                             (branch, len(bheads)))
+
         parent = repo.dirstate.parents()[0]
-        if len(heads) == 1:
+        if len(bheads) == 1:
+            if len(repo.heads()) > 1:
+                raise util.Abort(_("branch '%s' has one head - "
+                                   "please merge with an explicit rev") %
+                                 branch)
             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:
+        if parent not in bheads:
             raise util.Abort(_('working dir not at a head rev - '
                                'use "hg update" or merge with an explicit rev'))
-        node = parent == heads[0] and heads[-1] or heads[0]
+        node = parent == bheads[0] and bheads[-1] or bheads[0]
     return hg.merge(repo, node, force=force)
 
 def outgoing(ui, repo, dest=None, **opts):