Show calculated members in value prompt

[cognos05]

Hi,

I am using Cognos Dimensional reporting . I have a product dimension which has members at different levels.

All I am doing is grouping certain products like prd1a+prd1b as prdAb , then I have prd1c, prd1d members.

My promt should show values like prdAB then prd1c,prd1d.

my expression for prompt query was

set(prdAB which is a calculated data item, prd1c, prd1d )

But my report gets crashed while running the report RSV-SRV-0040 . If I remove the calculated member from my set expression(prdAB) then the report runs fine.
Cant I use a calculated member from product dimension in a set query feeded to value prompt ?

How can I achieve this.

Any help is appreciated !!

Thanks,