ssrs conditional runningvalues = includes all recs?

[BruceBussell]

How do you get ssrs conditional running values that only include items that have Shipped.Value = True

=IIF(Fields!Shipped.Value = True, RunningValue(Fields!Order_Amount.Value, Sum, "Group1"), RunningValue(Fields!Order_Amount.Value,0 , "Group1"))

Still does not work, as it includes Both True and False value records in its runningvalue expression.