To reproduce:

  $ hg init repo
  $ hg export -R repo -r 'not all()'

** unknown exception encountered, please report by visiting
**  http://mercurial.selenic.com/wiki/BugTracker
** Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40) [GCC 4.4.5]
** Mercurial Distributed SCM (version 2.1.2+13-913be8c5139c)
** Extensions loaded: foo, mq, patchbomb, convert, transplant, fetch, churn,
keyword, nearest, inout, rebase, relink
Traceback (most recent call last):
  File "/home/thomas/bin/hg", line 38, in <module>
    mercurial.dispatch.run()
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 27, in run
    sys.exit((dispatch(request(sys.argv[1:])) or 0) & 255)
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 64, in dispatch
    return _runcatch(req)
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 87, in _runcatch
    return _dispatch(req)
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 685, in _dispatch
    cmdpats, cmdoptions)
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 467, in
runcommand
    ret = _runcommand(ui, options, cmd, d)
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 739, in
_runcommand
    return checkargs()
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 693, in checkargs
    return cmdfunc()
  File "/home/thomas/hg/repos/tah/mercurial/dispatch.py", line 682, in <lambda>
    d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
  File "/home/thomas/hg/repos/tah/mercurial/util.py", line 456, in check
    return func(*args, **kwargs)
  File "/home/thomas/hg/repos/tah/mercurial/extensions.py", line 139, in wrap
    util.checksignature(origfn), *args, **kwargs)
  File "/home/thomas/hg/repos/tah/mercurial/util.py", line 456, in check
    return func(*args, **kwargs)
  File "/home/thomas/hg/repos/tah/hgext/mq.py", line 3343, in mqcommand
    return orig(ui, repo, *args, **kwargs)
  File "/home/thomas/hg/repos/tah/mercurial/util.py", line 456, in check
    return func(*args, **kwargs)
  File "/home/thomas/hg/repos/tah/mercurial/commands.py", line 2425, in export
    opts=patch.diffopts(ui, opts))
  File "/home/thomas/hg/repos/tah/mercurial/cmdutil.py", line 533, in export
    revwidth = max([len(str(rev)) for rev in revs])
ValueError: max() arg is an empty sequence