conditional sum in rdlc

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reporting-services

Hi,

I want to add the following expression to a textbox in my rdlc report:

=Sum(iif(Previous(Fields!VWINKEL.Value)<>Fields!VWINKEL.Value,Fields!VAANTALNIEUW.Value,0))

But it seems to be impossible to use the Previous expression in an sum function. I get this error message:

"The Value expression for the textbox contains an aggregate function (or RunningValue or RowNumber functions) in the argument to another aggregate function (or RunningValue). Aggregate functions cannot be nested inside other aggregate functions."

How can i resolve this problem?