MDX parent set of a set

Hi,

I have a following kind of query:

WITH MEMBER [Measures].[ParentSum] AS ([Measures].[Amount], [Dim].[ComparisonHierachy].currentmember.parent)

SELECT {[Measures].[Amount], [Measures].[ParentSum]} ON 0,
NON EMPTY {( [Dim2].[Dim2].[Dim2])} ON 1
FROM [Cube]
WHERE {[Dim].[Dim].&[1])}

This works fine as long as the WHERE part points to only one member. If it is a set, I get an error for ParentSum. What do I need to change in order to get ParentSum to work for sets too?