mdx create member question

[foreveryoung]

I will try and make this as simple to understand as possible.

I want to create a calculated member that will allow me to use the result as part of a query.

The calculate member must contain the sum of a particular measure (measures.amount). This member must have already had two dimension filters applied.

Here is my example

with member [Measures].[GAElectiveOrdinary]
as
'(sum({[Admission Method].[Admission Method].&[3],[Discharge Destination].[Discharge Destination].&[Court]},[Measures].[Spells]))'

select [Ward].[Ward].members on rows,
[Measures].[GAElectiveOrdinary] on columns from [Inpatient]

The filters are as follows

({[Admission Method].[Admission Method].&[3],[Discharge Destination].[Discharge Destination].&[Court]}

when I run this I get an error saying

Members belong to different hierarchies in the function.

Does anyone know what I am doing wrong please? Do I need to reconstruct the mdx differently?

Thanks
David