Fixing existing translation credits fails with a traceback

Here's the traceback for fix_translation_credits.py run. It ran up to 40k/410k before failing like this.

Traceback (most recent call last):
  File "/srv/launchpad.net/production/launchpad/scripts/rosetta/fix_translation_credits.py", line 28, in ?
    script.lock_and_run()
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/services/scripts/base.py", line 290, in lock_and_run
    implicit_begin=implicit_begin, isolation=isolation)
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/services/scripts/base.py", line 248, in run
    self.main()
  File "/srv/launchpad.net/production/launchpad/scripts/rosetta/fix_translation_credits.py", line 22, in main
    fixer.run()
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/translations/scripts/fix_translation_credits.py", line 98, in run
    DBLoopTuner(loop, 5).run()
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/canonical/launchpad/utilities/looptuner.py", line 107, in run
    self.operation(chunk_size)
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/translations/scripts/fix_translation_credits.py", line 74, in __call__
    potmsgset.setTranslationCreditsToTranslated(pofile)
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/translations/model/potmsgset.py", line 1062, in setTranslationCreditsToTranslated
    lock_timestamp=datetime.datetime.now(pytz.UTC))
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/translations/model/potmsgset.py", line 760, in updateTranslation
    lock_timestamp))
  File "/srv/launchpad.net/production/launchpad-rev-8648/lib/lp/translations/model/potmsgset.py", line 716, in _isTranslationMessageASuggestion
    assert (is_imported or is_editor or
AssertionError