SSAS/OLAP Client With Level-Specific Calculations
===

My SSAS data cube has a calculation dimension that includes a "Time Period" hierarchy, which is used to perform and present various time-based calculations (e.g., same time last year, YTD, moving averages, etc.).  Below is my code for a 4-quarter moving average.

    SCOPE([Calculation].[Time Period].&[4]); //4-Quarter Moving Average
     THIS =
        IIF(
                [Date].[Quarterly Calendar].CurrentMember.Level IS [Date].[Quarterly Calendar].[Quarter]
            AND
                Count(
                    NonEmpty(
                        LastPeriods(4, [Date].[Quarterly Calendar].CurrentMember.PrevMember)
                    )
                ) = 4,
            AGGREGATE(LastPeriods(4, [Date].[Quarterly Calendar].CurrentMember.PrevMember), [Calculation].[Time Period].&[1]) / 4,
            NULL
        );
    END SCOPE;

My problem is that because the moving average's value is always NULL at the Year level of the Quarterly Calendar hierarchy, the OLAP client (Excel in this case), hides the column corresponding to the four-quarter moving average, even when I expand the year level down into the quarter level.  Is there some hint or setting I need to set so that the 4-quarter moving average column displays (all the other columns in the Time Span hierarchy display as expected).