Delegate Search - not able to set on key of object

Hi

I have a BEx query with a SAP variable on 0MATERIAL (Material) that is taking forever (3mins) to search for a value in the Webi prompt screen.

0MATERIAL has 280,000 entries.

I am wanting to use the "Delegate search" functionality in the Universe Designer to speed up the search for values.

However, I am only able to make that setting on "LovMaterialBase" - the text of 0MATERIAL.

If I try to set "LovMaterial" to Delegate Search I get the below error message:

"Cannot set Delegate Search because the object has a reference in a custom list of values"

I have done a search for the above message but have not been able to find any thing relating to it.

This issue is also not limited to this specific universe/webi.  It appears for ALL of the universes/webis we use I am unable to set the key of ANY object to Delegate Search.

Users generally search by key so it is necessary to be able to make this sitting on the key.

Does anyone know what that message refers to?

Thanks

Anton