Combining Calculated Member with a Calculated Measure

This code is fake but illustrates my issue

WITH

SET someSet as FILTER(dimWhatever.someHierarchy.allmembers,Measures.Relevance<5)

MEMBER dimWhatever.someHierarchy.[Other] AS Aggregate(someSet)

MEMBER Measures.avgMeasure AS Measures.something/Measures.somethingElse

SELECT Measures.avgMeasure ON COLUMNS,

dimWhatever.someHierarchy.[Other] ON ROWS

FROM theCube

I get nulls for my avgMeasure... why?  I don't get nulls for Measures.something or Measures.somethingElse.

Thanks!

Ken