Sum(ReportItems!..) in Matrix

Hi , I have a detail field called Frames in matrix that does calculation with ReportItem and it'is the lowest group in the Matrix.

count(Fields!Subscription_No_.Value)*ReportItems!Textbox141.Value

I'd like to calculate Totals for Frames across parent group or whole matrix, for this fiels like sum(ReportItems!Frames.Value)

I'm getting error: The Value expression ...uses an aggregate function on a report item. Aggregate functions can be used only on report items contained in page headers and footers.

Pls help.