SUBTRACT MULTIPLE FIELDS

Hello I am trying to subtract multiple fields.  Here is what I am trying to do:

((payment amount) - (client fee) - (override) )/2

The main problem I am having is the override field has blanks so it does't recognize the formula.  Perhaps the field types need to be changed?

Payment amount - Currency Field
Client Fee - Currency Field
Override - Formula - Numeric

Thanks for your help.