--- a/mercurial/commands.py	Tue May 11 17:13:35 2010 -0500
+++ b/mercurial/commands.py	Mon May 10 08:43:36 2010 +0200
@@ -717,6 +717,10 @@
     """
     extra = {}
     if opts.get('close_branch'):
+        if repo['.'].node() not in repo.branchheads():
+            # The topo heads set is included in the branch heads set of the
+            # current branch, so it's sufficient to test branchheads
+            raise util.Abort(_('can only close branch heads'))
         extra['close'] = 1
     e = cmdutil.commiteditor
     if opts.get('force_editor'):