Currently it just crashes:

  quark % hg export 'wdir()'
  # HG changeset patch
  # User foo
  # Date 1481214154 0
  #      Thu Dec 08 16:22:34 2016 +0000
  ** Unknown exception encountered with possibly-broken third-party extension lz4revlog
  ** which supports versions 3.9 of Mercurial.
  ** Please disable lz4revlog and try your action again.
  ** If that fixes the bug please report it to the extension author.
  ** Python 2.7.12 (default, Oct  4 2016, 02:38:46) [GCC 6.1.1 20160501]
  ** Mercurial Distributed SCM (version 4.0+199-a31634336471)
  ** Extensions loaded: rebase, lz4revlog
  Traceback (most recent call last):
    File "/home/quark/hg/hg", line 45, in <module>
      mercurial.dispatch.run()
    File "/home/quark/hg/mercurial/dispatch.py", line 61, in run
      sys.exit((dispatch(request(sys.argv[1:])) or 0) & 255)
    File "/home/quark/hg/mercurial/dispatch.py", line 127, in dispatch
      ret = _runcatch(req)
    File "/home/quark/hg/mercurial/dispatch.py", line 217, in _runcatch
      return callcatch(ui, _runcatchfunc)
    File "/home/quark/hg/mercurial/dispatch.py", line 226, in callcatch
      return func()
    File "/home/quark/hg/mercurial/dispatch.py", line 206, in _runcatchfunc
      return _dispatch(req)
    File "/home/quark/hg/mercurial/dispatch.py", line 906, in _dispatch
      cmdpats, cmdoptions)
    File "/home/quark/hg/mercurial/dispatch.py", line 651, in runcommand
      ret = _runcommand(ui, options, cmd, d)
    File "/home/quark/hg/mercurial/dispatch.py", line 914, in _runcommand
      return cmdfunc()
    File "/home/quark/hg/mercurial/dispatch.py", line 903, in <lambda>
      d = lambda: util.checksignature(func)(ui, *args, **cmdoptions)
    File "/home/quark/hg/mercurial/util.py", line 1041, in check
      return func(*args, **kwargs)
    File "/home/quark/hg/mercurial/commands.py", line 3783, in export
      opts=patch.diffallopts(ui, opts))
    File "/home/quark/hg/mercurial/cmdutil.py", line 1171, in export
      single(rev, seqno + 1, fp)
    File "/home/quark/hg/mercurial/cmdutil.py", line 1152, in single
      write("# Node ID %s\n" % hex(node))
  TypeError: b2a_hex() argument 1 must be string or buffer, not None



It'll be nice to make it work.