Improper use of radio buttons results in 'illegal choice'

This is perhaps more a warning to users than a bug fix request, but it does seem different to other similar bug reports so I'm posting it.

I had a yes/no question that was not mandatory. 'Would you take part again?'

If not ticked this resulted in 'illegal choice' error message. This is correct as a field that represents two mutually exclusive choices should be mandatory - however the error message / validation is presented to the wrong person - it should be given to the person creating the form and not to the person filling it out.

A more efficient way of doing it would be to use a check box response to the question - 'tick here if you'd like to do this again' - left optional.

The only way to get a check box is to check the 'multiple' box.