Permissions Issue with Connected Lookup Field


We have a user trying to add a connected lookup field to a list, and when they set it all up and click OK they get an "Access Denied" error page. The destination list (with the connected lookup field) is on a subsite within the same site collection, and the source list is on the top-level site. The user is in the Owners group for both sites and therefore has Full Control on both lists (the one he's adding to as well as the one he's trying to pull a value from). I can do the same thing with my administrative account with no error. What effective permissions are necessary for this feature to work?