SSRS Expression Error, how to debug?

Question

Hi,

Is there any way to properly debug a VB cell expression within an SSRS report?

I'm getting an #error coming up when I'm convinced the data is fine for the expression.

The expression is:

"Total Opportunities ("+SUM(IIF(Fields!OpportunityEstValue.Value > 0, 1, 0)).ToString()+")"

Within the data being supplied the OpportunityuEstVal is produced with a case statement to be either the value or 0 every time so there's no possiblity of it being NULL. I've also tried adding NOTHING comparissons but still I receive an #error statement.

Any advice on how I can determine the source of the #error?