diff -r cf8b8f62688a -r f6e961c0155b mercurial/commands.py
--- a/mercurial/commands.py	Mon Jun 25 21:23:24 2007 -0500
+++ b/mercurial/commands.py	Tue Jun 26 15:28:17 2007 +0200
@@ -169,6 +169,9 @@
     if not rev:
         rev = node
 
+    if not rev:
+        raise util.Abort(_("please specify a revision to backout"))
+
     cmdutil.bail_if_changed(repo)
     op1, op2 = repo.dirstate.parents()
     if op2 != nullid: