def serialize_unregistered(fullname, email):
    user = auth.get_user(email=email)
    if user is None:
        serialized = {
            'fullname': fullname,
            'id': None,
            'registered': False,
            'active': False,
            'gravatar': gravatar(email, use_ssl=True,
                                 size=settings.PROFILE_IMAGE_MEDIUM),
            'email': email,
        }
    else:
        serialized = add_contributor_json(user)
        serialized['fullname'] = fullname
        serialized['email'] = email
    return serialized
Check that filter_horizontal is a sequence of field names.
dataset/ETHPy150Open CenterForOpenScience/osf.io/website/profile/utils.py/serialize_unregistered--crossover with--dataset/ETHPy150Open django/django/django/contrib/admin/checks.py/BaseModelAdminChecks._check_filter_horizontal