Join on Expression gives an error

I have a table Pax (PaxId, PaxName) and a query qryPaxRooms (RoomPaxId, PaxName). RoomPaxId in the query is an expression with Nz(ResRooms.PaxId,ResPaxid) AS RoomPaxId. So if a reservation has no rooms yet assigned RoomPaxId will have the PaxId of the reservation otherwise the PaxId of the Room Owner (might be different). Now if I try to join table qryPaxRooms with a different table Invoices (InvID, InvDate, PaxId) on qryPaxRooms.RoomPaxId with Invoices.PaxId I get a type mismatch error. Is it not possible to do a join on expressions? Is there a different solution? Thank you so much for any help! Regards Michael