today() generating error when used in an expression

I am using today() as part of an expression to generate a conditional running total.  It is producing an error in the report output.

=RunningValue(
IIF(
(not(IsNothing(Fields!PPAP_Sent_Date.Value)) and (Fields!PPAP_Sent_Date.Value <= Fields!PPAP_Request_Date.Value)
or(isnothing(Fields!PPAP_Sent_Date.Value) and (Fields!PPAP_Request_Date.Value > Today())))
,Fields!Item.Value,0
),sum,"Month" )