Donation API error

Hi I'm setting up a form on an external site to submit donations to one of our donation forms. When I submit the external form I get the error message "You must enter Yes or No" (see XML below), but I don't know where this is coming from. I used a copy of a donation form that was created by someone else in our organization, so what I'm wondering is if there is an easy way to find out where this error message is coming from? The same error message doesn't seem to get triggered when filling in the form directly.

- <donationResponse xsi:schemaLocation="http://convio.com/crm/v1.0 http://service.convio.net/xmlschema/crm.public.v1.xsd" xmlns="http://convio.com/crm/v1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> - <errors> <code>101</code> <message>Error: There was a problem encountered while processing your donation.</message> <reason>FIELD_VALIDATION</reason> <pageError>There was a problem processing your request. Please see below.</pageError> <fieldError>You must enter Yes Or No</fieldError> </errors> </donationResponse>

The new external form we are testing is :

Thanks,

Andrew