IIf Statement - Different Problem With RDLC

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VB

Visual-Studio

VS2010

RDLC

Hi, Iam using VS2010, while generating RDLC report, Iam using IIf statement for calculations.
But I found a typical problem which is very different.
Because from the Code.Functions for calculation, simply it's working fine, when I give the
below command directly

=Code.AddMyNet(Sum(ReportItems!itm_value.Value))

But I wish to give like the below command, It's giving error "local report processing"

=IIf((ReportItems!Textbox15.Value ="CARRY FORWARDED "), Code.AddMyNet(Sum(ReportItems!itm_value.Value)), Nothing)

Actually Function AddMyNet() used to calculate the pagewise running value total.

And hence any guidances will be helpful

Thanks & Regards
Paramu