SSRS Report Builder Sum If #Error

I am driving myself crazy with this.  I cannot get sum if to work.  I have two datasets in the report and the fields for the sum if are both in DataSet1.

Originally I attempted this:

=sum(iif(month((Fields!ScheduledShipDate.Value, "DataSet1"))=month(today()),(Fields!ExtendedPrice.Value, "DataSet1"),0))

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That was giving me  a "The Value expression for the textbox 'TextBox1' uses an aggregate expression without a scope.  Then I found that you can specify the scope within the Sum function so I changed it to this:

=sum(iif(month(CDate(Fields!ScheduledShipDate.Value))=month(today()),CDec(Fields!ExtendedPrice.Value),0),"DataSet1")

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Now the report will run but the textbox just displays "#Error".  I have no clue what the error is or how to debug it.  It seems to me that this should surely be working.

FYI, i removed and year(..)=year(today()) just to keep it as simple as possible while i was testing it.