Can't uncheck all checkboxes after one is checked

I am using editablefields to present a single field as a series of checkboxes, by allowing an unlimited number of values.

It works fine except that on a new node where boxes are still all unchecked, if you click a checkbox and then change your mind and try to uncheck it, you'll get an error message:
error : An illegal choice has been detected. Please contact the site administrator.
This problem only manifests itself when you are trying to uncheck the last of any checkboxes that were checked. That is, if you check 3 boxes and then uncheck 2, you are ok. It's only when you try to uncheck the last box that you get the error.

My nasty work-around for now was to add an extra box which is enforced (via hook_nodeapi) to always be checked. But this is obviously not a great solution.

Dan