shmk on "[Plugin: Contact Form 7] Acceptance checkbox validation"

Hello,
I'm tring to have the acceptance checkbox returning the form validation instead of disabling the submit button.

I've added "acceptance_as_validation: on" on additional notes but without positive results.

I'm on CF7 3.5.4 and WP 3.7.1,
any hint?

http://wordpress.org/plugins/contact-form-7/