SQL Query as Attribute value

Hi,

I have a little problem with an newly created custom attribute. This attribute has presentation SingleSelect and the attribute values should be selected via an SQL query. So far so good.

This attribute is displayed in a WET. When I open these WET I get an error messages with the following text: Could not expand values for attribute <ATTRIBUTE NAME>.

The used SQL query

SELECT distinct REP_NAME 
   FROM MC_REPOSITORY
   WHERE REP_NAME is not null and (LENGTH(REP_NAME) = 6)

When we use the following SQL Query everything works fine.

SELECT distinct avalue 
   FROM MXIV_ENTRIES 
   WHERE attrname='ACCOUNTADS'

My problem now is that I don't understand why my query from MC_REPOSITORY doesn't work but the other works fine.

Best regards

Lars