Crystal and .csv table linking

I've linked a .csv file as my table in crystal. I have a debit, credit, and difference colum in the file which shows in the crystal report, except where anything has a negative value. I've tried creating calculted fields to produce the credit balance in crystal, which worked in my credit column. However, if I create a formula Debit-Credit where my debit might be a zero balance, my result is blank in the difference column. Anyone have any ideas? I've tried a running total as well with no luck.