How to avoid self-approval of performance goals

Hi Folks,

We have a requirement where the performance goals added/updated by the employees should be subjected to approval of Line Manager and Dept. Head (AOR). In the case where Dept. head is submitting the performance goals the request should only be sent to line manager, and the approval request should not come back to the dept. head. We tried using the "Prohibit self-approval by users named in these payload attributes" functionality and added the expression: /task:task/task:payload/ns2:getRequestorInformationByUserNameResponse/ns2:result/ns9:UserId

But the above rule failed. Appreciate if someone could let us know where we went wrong.