'An illegal choice has been detected..' when submitting a form.

I was getting the message An illegal choice has been detected. Please contact the site administrator. when submitting forms, and more annoyingly when doing an AJAX image upload on a form.

The problem is caused by the radio buttons that I have on my form.  They are set up without a default value but as required.  I believe the issue is that if a radio group dosen't have a value set, it doesn't existing within the DOM for the form in question, therefore leading to an error on the form checking.

The work-around for me was to change the field type from radio group to select list.  Another workaround would be to have a default value.