Checkbox form field on modular page not working

I have got a frontend form working, but adding a checkbox field throws validate errors on the first two required fields, removing the checkbox field lets work again like a charm. Using a radion button field does work instead, but not a checkbox (Grav v1.1.0-rc.3).

• name: confirmation
  label: 'I want to get a confirmation'
  type: checkbox

Tracking on https://github.com/getgrav/grav/issues/933