How to resolve "Type Mismatch error" in MDX query

Question

I am struggling with the Type Mismatch error when I query the cube in SSAS. The MDX query is shown below:

WITH 
    MEMBER [MEASURES].[TopNFilter] AS 
        IIF
        (
            Count((EXISTING Exists([DimClient].[Client].CurrentMember,TopNSet))) > 0,
            1,
            -1
        ) 
SELECT 
    {
        [MEASURES].[Volume],
        [TopNFilter]
    } ON COLUMNS,
    {TopNSet} ON ROWS
FROM [Cube];

How can I fix this error

Answer 1

Sorry for the brevity but the engine will not know the type of this word:

TopNSet

The rest of the syntax looks like valid MDX