TypeError: 'NoneType' object is not iterable

[sentry-io]

Sentry Issue: ROGUE-SCHOLAR-2

TypeError: 'NoneType' object is not iterable
(32 additional frame(s) were not displayed)
...
  File "citeproc/model.py", line 785, in render_single_date
    parts = self.parts(date, show_parts, context)
  File "citeproc/model.py", line 879, in parts
    part_text = part.render(date, context)
  File "citeproc/model.py", line 95, in render
    return self.markup(self.process(*args, **kwargs))
  File "citeproc/model.py", line 909, in process
    and self.get_locale_option('limit-day-ordinals-to-day-1')
  File "citeproc/model.py", line 116, in get_locale_option
    for locale in self.get_root().locales: