An illegal choice has been detected. Please contact the site administrator.

Hi!
Thanks for this awesome module.
I have noticed that when a invitee visits the invitation page and click on the "Reply" button, without making a choice in the "will-you-reply" boxes, he/she gets an error message that says "An illegal choice has been detected. Please contact the site administrator."

I guess a more suitable error message would be something like "Please reply by enter a choice if you would like to attend..."

I first goggled this error message and found that other modules shouted out this error message as well, so I guess that this is something that's created by Drupal and not by the RSVP module it self?