Invalid entry error when entering a negative number

I have a numeric-currency field that I am using to adjust a total field. If I edit the record to enter a negative number I get an error Invalid entry. But I can enter a positive number, save and reopen the record, edit the field to add a "-" then it works. So why won't it accept the "-" on the first attempt? Why must I enter the positive value, re-edit, add the "-" in order to make it work?