How to update the Shipment Stop table through agent of type 'Shipment Stop'

I was trying to update the Shipment_Stop table through agent of type 'Shipment Stop' and event 'Shipment- Stop created' but unable to as the $GID is union key (Shipment_gid + Stop_num) in this case how do we update the Shipment Stop table. Can you please help

I am getting the below error when trying to

Shipment Stop TEST.27115x2 - error

Error Agent TEST.TOTAL_MILES_CALCULATION_PER_STOP failed

Cannot assign null value to variable $FETCH_STOP_NUM

Note though the shipment has Stop_Num, I am not able to fetch also not able to pass the $gid as it is a combination key.