TypeError: 'NoneType' object is not subscriptable

Sentry Issue: GWCELERY-TS

TypeError: 'NoneType' object is not subscriptable
  File "celery/app/base.py", line 487, in run
    return task._orig_run(*args, **kwargs)
  File "gwcelery/tasks/gracedb.py", line 32, in wrapper
    return f(*args, **kwargs)
  File "gwcelery/tasks/gracedb.py", line 289, in update_superevent
    coinc_far=coinc_far)
  File "gracedb_sdk/api/base.py", line 39, in update
    return self.create_or_update(key, **kwargs)
  File "gracedb_sdk/api/events.py", line 60, in create_or_update
    category = SUPEREVENT_CATEGORIES[kwargs['preferred_event'][0]]