Formula and blank fields bug?

Formula and blank fields bug?

I have a table with 3 columns. Sometimes 1 of the 3 columns can be blank.
 
The third column is a formula column where I basically do "Sales - Costs" to get "Profit". But if the Sales or Costs column is blank on a ny row the Profit column stays blank.
 
Where it should be:
 
Sales | Costs | Profit
----------------------------
100  | 50 | 50
100 | 0 | 100
 
 
Any solution to this behaviour?