Cannot save events with participants

Sentry Issue: CONCREXIT-1P

AssertionError: Cannot filter a query once a slice has been taken.
(11 additional frame(s) were not displayed)
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  File "django/contrib/admin/options.py", line 1561, in _changeform_view
    self.save_model(request, new_object, form, not add)
  File "django/contrib/admin/options.py", line 1088, in save_model
    obj.save()
  File "events/models.py", line 398, in save
    self.participants.filter(member__isnull=False)
  File "django/db/models/query.py", line 892, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "django/db/models/query.py", line 904, in _filter_or_exclude
    "Cannot filter a query once a slice has been taken."