Why is BPM rule for AP invoice approval giving an error that Invoice Line.Requester Name is invalid?

Summary: We are needing to add a rule to our BPM workflow for AP invoice approval that requires our Service invoices (2 way match) to go to the PO requester for approval before payment. When we try to add that to our ruleset, we get an error that the Invoice Line.Requester Name is invalid. It validates if I use "workflowsystem" instead of "Invoice Line.Requester Name", but then it doesn't go to the requester. It also will validate if I change one of the conditions so that it doesn't use "at least one". But if I do