traceback during populate_daily_metrics

found this traceback in the logs:
Traceback (most recent call last):
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/tasks.py", line 110, in populate_daily_metrics
    force_update=force_update)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/celery/local.py", line 188, in __call__
    return self._get_current_object()(*a, **kw)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/celery/app/trace.py", line 439, in __protected_call__
    return orig(self, *args, **kwargs)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/celery/app/task.py", line 428, in __call__
    return self.run(*args, **kwargs)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/tasks.py", line 56, in populate_single_cdm
    course_id).load(date_for=date_for, force_update=force_update)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/pipeline/course_daily_metrics.py", line 335, in load
    data = self.get_data(date_for=date_for)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/pipeline/course_daily_metrics.py", line 279, in get_data
    date_for=date_for)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/pipeline/course_daily_metrics.py", line 261, in extract
    course_id, date_for,)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/pipeline/course_daily_metrics.py", line 178, in get_average_days_to_complete
    days_to_complete = get_days_to_complete(course_id, date_for)
  File "/edx/app/edxapp/venvs/edxapp/src/figures/figures/pipeline/course_daily_metrics.py", line 164, in get_days_to_complete
    days.append((cert.created_date - ce[0].created).days)
  File "/edx/app/edxapp/venvs/edxapp/local/lib/python2.7/site-packages/django/db/models/query.py", line 289, in __getitem__
    return list(qs)[0]
IndexError: list index out of range
appsembler / figures

traceback during populate_daily_metrics #231
