The sentry.utils.social_auth.create_user_if_enabled module calls create_user_if_enabled, which in turn runs the Django social auth function.  If no username is set, it dies:

```
def create_user(backend, details, response, uid, username, user=None, *args,
                **kwargs):
    """Create user. Depends on get_username pipeline."""
    if user:
        return {'user': user}
    if not username:
        return None

```