This is telling you that there is something in the formula that Crystal can't find. Does the prompt {?My Parameter} exist and have a value? Are you correctly setting the parameter values in your code? What is the data type of {?My Parameter}? You're treating it as if it were a number, but is the parameter actually a number?
I would edit the report and go to wherever this particular If statement is located. Then verify the formula to make sure that Crystal understands it. The program will give you more info about where in the formula the problem is.
-Dell