Checkbox custom field can not be unset

A custom field is created which consists of one or more check boxes. Once any checkboxes are set for that field, it can never be completely un-set.

If you attempt to un-check all boxes, no change is made to the field at all. All previously checked boxes remain 'checked.'

The only way for changes to the field to 'stick' is for a minimum of one box to be checked.