I have a table for holding client responses to a questionnaire. 10 of the fields in this table are List Boxes with options "Yes", "No", and "Blank/Other". The "Blank/Other" option is supposed to be reserved for forms handed in where the field has been intentionally left blank. If the form has not been handed in at all, I want the fields to just be NULL. The problem is that if someone accidentally fills out information for a row that should all be NULL, there is no way to correct it. I have tried adding NULL (no quotations) as an option in the list box, but this does not work. I have also tried adding "" as an option, but no luck there either.
tl;dr: How do I add an option in a list box to leave a field NULL?