Microsoft Access Checkboxes are behaving like radio buttons

I have a form in MS Access that I am developing. I have two checkboxes that are not mutually exclusive - CANCELLED and SUBMITTED. The checkboxes are not bound to any data table or query. I want the checkboxes to behave like you would expect checkboxes to behave - I can select them independently - zero, one or two of the controls can be checked. Very oddly though, they are behaving as if they were grouped radio buttons - if I choose one checkbox the other is unchecked. Like a radio button, I can't unselect it at all. This all seems related to the OptionValue property. I tried unsetting the value, but Access won't allow that and gives an error! I have also tried setting them to different values including 0 or -1 hoping that was a magic value to indicate that my checkbox should behave like a checkbox. How do I get checkboxes to behave like checkboxes?

Access checkbox property editor

Checkboxes should not be mutually exclusive options