--- a/mercurial/commands.py	Sat May 23 20:38:08 2009 +0900
+++ b/mercurial/commands.py	Sat Jun 13 22:42:13 2009 +0200
@@ -339,6 +339,8 @@
 
     if command:
         commandpath = util.find_exe(command)
+        if commandpath is None:
+            raise util.Abort(_("cannot find executable: %s") % command)
         changesets = 1
         try:
             while changesets: