SSRS runtime error - The specified operation is not valid

I found a similar post here, but it didn't solve my problem. so i re-post it.

i develop a report with vs2008. i have sql 2008 r2 on my local machine. when i preview the report on my local machine all fine. but when i deployed the report to sql 2008 ssrs server, i got "#Error" in my subtotal field. i opened the log file and found the error message:

The specified operation is not valid. , ;
 Info: Microsoft.ReportingServices.ReportProcessing.ReportProcessingException: The specified operation is not valid.

i used a formular in the expression:

=Sum(IIf(Fields!flag.Value="S",Fields!Size.Value,0))

can someone help?

thanks