Your result type is not a number or a date

This is probably a stupid question, but why is my result not a number in the following If statement?

IF({Course Order}>0,{Cost-1st Attendee}+{Cost-2nd Attendee}+{Cost-Additional Attendees}, “”)

It actually works, but I can’t format it, and the Summary fields don’t work. When I click on the Formatting tab, I get the following message.

“Your result type is not a number or a date. Formatting options are currently only available if your result type is a number or a date.”

How can I get the results to be formatted as currency (I assume that fix will make the Summary fields work)?

OK, as I said, stupid question. Replaced the double quotes (that dictate text) with a zero, and all is well with the world.