open-metrics fail to render

Requesting /api/1.0/open-metrics/ results in {"title": "Error"} on both debusine.f.c and debusine.d.n. The upshot is that this is not an internal server error and it's not spamming our mail log and it gives a traceback in the errors.log:

Server exception. status_code: 400 detail: None traceback:  Traceback (most recent call last):  File "/usr/lib/python3/dist-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/server/views/open_metrics.py", line 90, in get
    data = openmetrics.generate_latest(registry())  # type: ignore
                                       ^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/server/views/open_metrics.py", line 74, in registry
    registry.register(DebusineCollector())
  File "/usr/lib/python3/dist-packages/prometheus_client/registry.py", line 40, in register
    names = self._get_names(collector)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/prometheus_client/registry.py", line 80, in _get_names
    for metric in desc_func():
  File "/usr/lib/python3/dist-packages/debusine/server/open_metrics.py", line 119, in collect
    yield self.count_user_identities()
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/debusine/server/open_metrics.py", line 305, in count_user_identities
    (str(int(stat["user__is_active"])), stat["issuer"]),
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'