$ hg backout
** unknown exception encountered, details follow
** report bug details to http://www.selenic.com/mercurial/bts
** or mercurial@selenic.com
** Mercurial Distributed SCM (version 0.9.4)
Traceback (most recent call last):
  File "/usr/bin/hg", line 11, in <module>
    mercurial.commands.run()
  File "/usr/lib64/python2.5/site-packages/mercurial/commands.py", line 3110, in run
    sys.exit(dispatch(sys.argv[1:], argv0=sys.argv[0]))
  File "/usr/lib64/python2.5/site-packages/mercurial/commands.py", line 3107, in dispatch
    return cmdutil.runcatch(u, args, argv0=argv0)
  File "/usr/lib64/python2.5/site-packages/mercurial/cmdutil.py", line 37, in runcatch
    return dispatch(ui, args, argv0=argv0)
  File "/usr/lib64/python2.5/site-packages/mercurial/cmdutil.py", line 364, in dispatch
    ret = runcommand(ui, options, cmd, d)
  File "/usr/lib64/python2.5/site-packages/mercurial/cmdutil.py", line 417, in runcommand
    return checkargs()
  File "/usr/lib64/python2.5/site-packages/mercurial/cmdutil.py", line 373, in checkargs
    return cmdfunc()
  File "/usr/lib64/python2.5/site-packages/mercurial/cmdutil.py", line 356, in <lambda>
    d = lambda: func(ui, repo, *args, **cmdoptions)
  File "/usr/lib64/python2.5/site-packages/mercurial/commands.py", line 176, in backout
    node = repo.lookup(rev)
  File "/usr/lib64/python2.5/site-packages/mercurial/localrepo.py", line 363, in lookup
    n = self.changelog._match(key)
  File "/usr/lib64/python2.5/site-packages/mercurial/revlog.py", line 768, in _match
    if len(id) == 20:
TypeError: object of type 'NoneType' has no len()