ValueError: The given username must be set

Sentry Issue: CONCREXIT-1N

ValueError: The given username must be set
(20 additional frame(s) were not displayed)
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  File "registrations/signals.py", line 12, in post_payment_save
    services.process_payment(instance)
  File "registrations/services.py", line 414, in process_payment
    member = _create_member_from_registration(registration)
  File "registrations/services.py", line 286, in _create_member_from_registration
    last_name=registration.last_name
  File "django/contrib/auth/models.py", line 151, in create_user
    return self._create_user(username, email, password, **extra_fields)
  File "django/contrib/auth/models.py", line 140, in _create_user
    raise ValueError('The given username must be set')