Not null check on indexed attribute

we have a vertex and below query we are trying to find all RFS with name as null, it is working fine.

result = select rfs from RFS:rfs where rfs.name == “”;

But for the same check on indexed attribute cust_id it is not giving results even though the value is empty. cust_id is indexed

result = select rfs from RFS:rfs where rfs.cust_id== “”;

Please help how to write null check on indexed attribute.