New Post: Adding a checkbox during checkout, is it allowed?

Hello,
I want to make users accept ToS during checkout process.

So I thought adding an extra checkbox and a rfvalidator on the checkout template - similarly to adding a custom field but without databind property - would do the work but it didn't. Instead i got

System.Web.HttpException: Control 'chkAcceptTerms' referenced by the ControlToValidate property of 'valChkAcceptTerms' cannot be validated.

So it seems that a checkbox cannot be validated by a rfvalidator but it requires a custom validator. Can you please help?

Thanks