File "indico/modules/events/registration/util.py", line 516, in modify_registration
    _set_data(form_item, value)
  File "indico/modules/events/registration/util.py", line 463, in _set_data
    attrs = field.field_impl.process_form_data(registration, value, data_by_field[field.id],
  File "indico/modules/events/registration/fields/base.py", line 322, in process_form_data
    if billable_items_locked and old_data.price != self.calculate_price(value, new_data_version.versioned_data):
  File "indico/modules/events/registration/models/registrations.py", line 906, in price
    return self.field_data.field.calculate_price(self)
  File "indico/modules/events/registration/models/form_fields.py", line 113, in calculate_price
    return self.field_impl.calculate_price(registration_data.data, registration_data.field_data.versioned_data)
  File "indico/modules/events/registration/fields/accompanying.py", line 87, in calculate_price
    return Decimal(str(versioned_data.get('price', 0))) * len(reg_data)