Date Error in Measure

Hi,

The Measure below is producing the error message in yellow below it. 

Cases_Reopened = 
    VAR _lastdate = 
            CALCULATE(MAX('BAU Closures'[Month].[Date]),
            FILTER('BAU Closures','BAU Closures'[Cases Reopened] <> BLANK() )
            )
    RETURN
        CALCULATE(
                MAX('BAU Closures'[Cases Reopened]),
                FILTER ('BAU Closures','BAU Closures'[Month] = _lastdate)
        )

I've hidden the columns in the excel below. I just want to show the last Value in the Cases Reopened (V) column that is Not Blank.

BAU Closures Month Date is Col A below.

Any ideas why this is happening and how to fix?

Thanks,