SSRS cannot aggregate a variable

Question

I am trying to create a sum value of a variable in SSRS. The variable is used to calculate the month previous to the one selected to compare items in the 2 months.

      |Month N-1 | Month N
Item X|    25    |   45

This is the code to calculate the previous month:

=iif(Parameters!Month.Value = 1, 12, Parameters!Month.Value - 1)

This is the code to calculate if the item has is in this month and tally them up:

=sum(iif(Month(Fields!ReportDate.Value) = Variables!PriorMonth1.Value, 1,0))

However, I get this error upon running the report:

Contains a variable reference in the argument to an aggregate function (or RunningValue). Variable values cannot be used in aggregate functions.

Answer 1

You should not need a variable to do this.

I can't test this at the moment but I think this should work.

=SUM(
    IIF(
        Month(Fields!ReportDate.Value) = (Parameters!Month.Value + 2 MOD 12) -1,
        1, 
        0
    )
)

All I've done here is calc the previous month number directly in the expression.