After playing with this for 45 minutes, i gave up and asked. Five minutes later, the solution smacked me in the face. I just hadn't considered the idea of cancelled orders when there are no sales. (sometimes users can make code go places the developer wouldn't guess possible)

fortunately, ".NULL. - 1" and "1 - .NULL." both result in ".NULL.", so both cases can be caught with Isnull(expr1 - expr2) whether expr1 and/or expr2 are .NULL.

"Now I know why they say to get away from your computer every few hours"