Unknown error when starting backup

[Filippo Bonazzi]

Traceback (most recent call last):
  File "/usr/bin/duplicity", line 1359, in <module>
    with_tempdir(main)
  File "/usr/bin/duplicity", line 1342, in with_tempdir
    fn()
  File "/usr/bin/duplicity", line 1222, in main
    globals.archive_dir).set_values()
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 684, in set_values
    self.get_backup_chains(partials + backend_filename_list)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 807, in get_backup_chains
    map(add_to_sets, filename_list)
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 797, in add_to_sets
    if set.add_filename(filename):
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 94, in add_filename
    (self.volume_name_dict, filename)
AssertionError: ({10: u'duplicity-full.20111014T071207Z.vol10.difftar.gpg', 11: u'duplicity-full.20111014T071207Z.vol11.difftar.gpg', 12: u'duplicity-full.20111014T071207Z.vol12.difftar.gpg'}, u'duplicity-full.20111014T071207Z.vol12.difftar.gz')

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: deja-dup 20.1-0ubuntu0.1
ProcVersionSignature: Ubuntu 3.0.0-13.21-generic 3.0.6
Uname: Linux 3.0.0-13-generic i686
ApportVersion: 1.23-0ubuntu3
Architecture: i386
Date: Mon Oct 24 11:02:57 2011
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release i386 (20110427.1)
SourcePackage: deja-dup
UpgradeStatus: Upgraded to oneiric on 2011-10-13 (10 days ago)