diff -r d280774a7498 -r be367cbafe70 mercurial/commands.py
--- a/mercurial/commands.py	Thu Dec 27 23:55:39 2007 -0600
+++ b/mercurial/commands.py	Thu Dec 27 23:55:39 2007 -0600
@@ -196,11 +196,9 @@
         raise util.Abort(_("please specify a revision to backout"))
 
     cmdutil.bail_if_changed(repo)
+    node = repo.lookup(rev)
+
     op1, op2 = repo.dirstate.parents()
-    if op2 != nullid:
-        raise util.Abort(_('outstanding uncommitted merge'))
-    node = repo.lookup(rev)
-
     a = repo.changelog.ancestor(op1, node)
     if a != node:
         raise util.Abort(_('cannot back out change on a different branch'))