Criteria to display a report with no value entered in a field

Criteria to display a report with no value entered in a field

I have a form with categories, more than one can be checked at a time but I need a report showing those that have no category selected. I have tried a few things but none seem to give me the response I am looking for. I need to be able to go back and add categories to the ones I was not sure of when adding them initially.