Exporting subdirs with --per-file-timestamps fails

When I try to export a subdir from a branch with --per-file-timestamps, I get the following error:

$ ./bzr export --per-file-timestamps ../export-bzrlib/ bzrlib/export
bzr: ERROR: The file id "None" is not present in the tree <bzrlib.inventory.CHKInventory object at 0x1019b4690>.

The corresponding bzr.log entry:

Fri 2011-06-10 15:42:58 +0200
0.072 bazaar version: 2.4.0dev4
0.073 bzr arguments: [u'export', u'--per-file-timestamps', u'../export-bzrlib/', u'bzrlib/export']
0.121 looking for plugins in /Users/szilveszter/.bazaar/plugins
0.138 looking for plugins in /Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/plugins
0.154 looking for plugins in /Users/szilveszter/Work/bzr.repo/env/lib/python2.7/site-packages/bzrlib/plugins
0.158 encoding stdout as sys.stdout encoding 'UTF-8'
0.222 export version <InventoryRevisionTree instance at 1019e8410, <email address hidden>'>
0.556 Transferred: 0kB (0.0kB/s r:0kB w:0kB)
0.562 Traceback (most recent call last):
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/commands.py", line 930, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/commands.py", line 1130, in run_bzr
    ret = run(*run_argv)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/commands.py", line 691, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/commands.py", line 713, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/builtins.py", line 2967, in run
    per_file_timestamps=per_file_timestamps)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/export/__init__.py", line 162, in export
    per_file_timestamps, fileobj):
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/export/__init__.py", line 129, in get_export_generator
    force_mtime=force_mtime, fileobj=fileobj):
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/export/dir_exporter.py", line 99, in dir_exporter_generator
    mtime = tree.get_file_mtime(tree.path2id(relpath), relpath)
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/revisiontree.py", line 99, in get_file_mtime
    ie = self._inventory[file_id]
  File "/Users/szilveszter/Work/bzr.repo/bzr.dev/bzrlib/inventory.py", line 1897, in __getitem__
    raise errors.NoSuchId(self, file_id)
NoSuchId: The file id "None" is not present in the tree <bzrlib.inventory.CHKInventory object at 0x1019b4690>.

The problem lies here:

mtime = tree.get_file_mtime(tree.path2id(relpath), relpath)

When exporting a subdir, relpath will be relative to the subdir, but path2id needs a path relative to the branch root.