list of devices not in ncm

I'm trying to create a query to list the devices in NPM that are potential candidates to be added to NCM.

Something along

select caption from orion.nodes where nodeid not in (select corenodeid from ncm.nodes) and vendor='Cisco'

but I get "Object reference not set to an instance of an object"

Basically, I'm trying to build a list of "Not Licensed by NCM" but with extra qualifiers

Any hints?