Scope error and nested aggregate function error

Hi,

I'm working in RD 1.0, and am using two datasets in one report. 

I initially ran into a scope error on the following expression:

=SUM(IIF(Fields!POSTED.Value="Yes",Val(Fields!RewardDollars.Value), Val(0)))

...but after modifying the expression (see below) to define which DS I want the data from I got another (nested aggregate function) error.  The original error is still there too.

=SUM(IIF(First(Fields!POSTED.Value, "DataSet1")="Yes",First(Fields!RewardDollars.Value,"DataSet1"),Val(0)))

...And here is the error message:

"The Value expression for the textbox '' textbox87' contains an aggregate function (or RunningValue or RowNumber functions) in the arguement to another aggregate function (or RunningValue).  Aggregate functions cannot be nested inside other aggregate functions."

I'm not sure why this error would rear its head only after I define the scope, but there you have it.

Could someone help out?

Thanks

Jens