bzrlib.errors.ObjectNotLocked raised when calling _mark_native_config

This is probably a fallout from bug #793137 and is encountered on jubany.

bin/import-package --verbose --no-push --keep-temp --local-branches ubiquity
Traceback (most recent call last):
  File "bin/import-package", line 7, in <module>
    run_script()
  File "/srv/package-import.canonical.com/new/scripts/udd/scripts/import_package.py", line 1149, in run_script
    only_before=options.only_before))
  File "/srv/package-import.canonical.com/new/scripts/udd/scripts/import_package.py", line 1054, in main
    bstore, push, possible_transports=possible_transports)
  File "/srv/package-import.canonical.com/new/scripts/udd/scripts/import_package.py", line 623, in import_package
    use_time_from_changelog=True)
  File "/home/vila/.bazaar/plugins/builddeb/import_dsc.py", line 1266, in import_package
    timestamp=timestamp, file_ids_from=file_ids_from,
  File "/home/vila/.bazaar/plugins/builddeb/import_dsc.py", line 1204, in _import_native_package
    native=True, timestamp=timestamp,
  File "/home/vila/.bazaar/plugins/builddeb/import_dsc.py", line 1044, in import_debian
    timestamp = timestamp[0]
  File "/home/vila/.bazaar/plugins/builddeb/import_dsc.py", line 932, in _mark_native_config
    current_config.filename = os.path.join(self.tree.basedir,
  File "/usr/lib/python2.7/dist-packages/bzrlib/tree.py", line 758, in id2path
    return self.inventory.id2path(file_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/inventory.py", line 669, in id2path
    self._iter_file_id_parents(file_id)][:-1]))
  File "/usr/lib/python2.7/dist-packages/bzrlib/inventory.py", line 1946, in _iter_file_id_parents
    ie = self[file_id]
  File "/usr/lib/python2.7/dist-packages/bzrlib/inventory.py", line 1906, in __getitem__
    self.id_to_entry.iteritems([StaticTuple(file_id,)]).next()[1])
  File "/usr/lib/python2.7/dist-packages/bzrlib/chk_map.py", line 1047, in iteritems
    for node, node_filter in self._iter_nodes(store, key_filter=key_filter):
  File "/usr/lib/python2.7/dist-packages/bzrlib/chk_map.py", line 1189, in _iter_nodes
    for record in stream:
  File "/usr/lib/python2.7/dist-packages/bzrlib/groupcompress.py", line 1474, in get_record_stream
    orig_keys, ordering, include_delta_closure):
  File "/usr/lib/python2.7/dist-packages/bzrlib/groupcompress.py", line 1580, in _get_remaining_record_stream
    locations = self._index.get_build_details(keys)
  File "/usr/lib/python2.7/dist-packages/bzrlib/groupcompress.py", line 2147, in get_build_details
    self._check_read()
  File "/usr/lib/python2.7/dist-packages/bzrlib/groupcompress.py", line 2069, in _check_read
    raise errors.ObjectNotLocked(self)
bzrlib.errors.ObjectNotLocked: <bzrlib.groupcompress._GCGraphIndex object at 0x3a1a350> is not locked