Participant custom fields not shown on registration form

I just tried this on the Drupal sandbox:

• Create a custom field set for Participant (general, not for a specific event or specific type)
• Add a custom field: 'Wants to carpool'. Choose 'yes or no' as type, and make it required.
• Now try to register someone for an event.

The custom field is not shown, but you get an error message because the required field 'Wants to carpool' is not filled out.