fast-import-filter: cannot concatenate 'str' and 'NoneType' objects

$ rsync -avzHS rsync://svn.savannah.gnu.org/svn/grub/ /tmp/grub.repo/
[...]
$ bzr fast-export-from-svn /tmp/grub.repo/ /tmp/grub.fe
[...]
$ bzr fast-import-filter /tmp/grub.fe
bzr: ERROR: exceptions.TypeError: cannot concatenate 'str' and 'NoneType' objects

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 842, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 1037, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 654, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/rmh/.bazaar/plugins/fastimport/__init__.py", line 500, in run
    verbose, user_map=user_map)
  File "/home/rmh/.bazaar/plugins/fastimport/__init__.py", line 111, in _run
    return proc.process(p.iter_commands)
  File "/home/rmh/.bazaar/plugins/fastimport/processor.py", line 95, in process
    self._process(command_iter)
  File "/home/rmh/.bazaar/plugins/fastimport/processor.py", line 117, in _process
    handler(self, cmd)
  File "/home/rmh/.bazaar/plugins/fastimport/processors/filter_processor.py", line 128, in commit_handler
    self.parents[":" + cmd.mark] = parents
TypeError: cannot concatenate 'str' and 'NoneType' objects

bzr 2.0.1 on python 2.5.4 (Linux-2.6.26-libre2-2-amd64-x86_64-with-debian-squeeze-sid)
arguments: ['/usr/bin/bzr', 'fast-import-filter', 'fe']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'ca_AD.UTF-8'
plugins:
  bzrtools /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [2.0.1]
  cia /usr/lib/python2.5/site-packages/bzrlib/plugins/cia [1.0.0dev]
  cvsps_import /usr/lib/python2.5/site-packages/bzrlib/plugins/cvsps_import [unknown]
  fastimport /home/rmh/.bazaar/plugins/fastimport [0.9.0dev]
  launchpad /usr/lib/python2.5/site-packages/bzrlib/plugins/launchpad [2.0.1]
  netrc_credential_store /usr/lib/python2.5/site-packages/bzrlib/plugins/netrc_credential_store [2.0.1]
  rebase /usr/lib/python2.5/site-packages/bzrlib/plugins/rebase [0.5.4]
  svn /usr/lib/python2.5/site-packages/bzrlib/plugins/svn [1.0.0]

*** Bazaar has encountered an internal error. This probably indicates a
    bug in Bazaar. You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/bzr/+filebug
    including this traceback and a description of the problem.