can't import git repo with ',branch=debian'

When attempting to import a code repository for the software 'jabref', I get an unexpected error that looks like a bug in launchpad. Am I doing something wrong here? The only slightly unusual thing I did was to request ",branch=debian" in order to get a non-main branch from the Git repo.

http://launchpadlibrarian.net/275803269/jdpipe-jabref-debian.log

2016-07-30 08:07:12 INFO Starting job.
2016-07-30 08:07:12 INFO Getting exising bzr branch from central store.
2016-07-30 08:07:12 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2016-07-30 08:07:12 INFO [chan bzr SocketAsChannelAdapter] Opened sftp connection (server version 3)
2016-07-30 08:07:13 INFO 285 bytes transferred
2016-07-30 08:07:14 INFO Importing branch.
2016-07-30 08:07:16 INFO Counting objects: 118432, done. 0
2016-07-30 08:07:57 INFO finding revisions to fetch:generating index 0/118432
2016-07-30 08:08:27 INFO finding revisions to fetch:generating index 0/118432
2016-07-30 08:08:51 INFO finding revisions to fetch 1/3
2016-07-30 08:08:51 INFO
Traceback (most recent call last):
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/scripts/code-import-worker.py", line 96, in <module>
    sys.exit(script.main())
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/scripts/code-import-worker.py", line 91, in main
    return import_worker.run()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/lib/lp/codehosting/codeimport/worker.py", line 583, in run
    return self._doImport()
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/lib/lp/codehosting/codeimport/worker.py", line 737, in _doImport
    inter_branch.fetch(limit=revision_limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/bzrplugins/git/branch.py", line 722, in fetch
    self.fetch_objects(stop_revision, fetch_tags=fetch_tags, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/bzrplugins/git/branch.py", line 745, in fetch_objects
    determine_wants, self.source.mapping, limit=limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/bzrplugins/git/fetch.py", line 718, in fetch_objects
    limit)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/bzrplugins/git/fetch.py", line 484, in import_git_objects
    mapping.revision_id_foreign_to_bzr)
  File "/srv/importd.launchpad.net/production/launchpad-rev-18169/bzrplugins/git/mapping.py", line 334, in import_commit
    raise UnknownCommitExtra(commit, [item[0] for item in commit.extra])
bzrlib.plugins.git.errors.UnknownCommitExtra: Unknown extra fields in <Commit e0380b795f649afaf6cd708abc5bf2680c8a01c0>: ['gpgsig'].
Import failed:
Traceback (most recent call last):
Failure: twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended with exit code 1.