AP Invoice Import Callback Integration erroring with Primary Identifier: undefined

We have AP Invoice Import integration that is successfully executed with invoices imported but the Callback integration ends in error : Primary Identifier: undefined.

We had followed all the per-requisities steps stated by Oracle like creating CSF key , Importing certificates ,having same user in ERP and OCI .

The callback integration does basic stuff like mailing the logs and moving the source file to archive , this fails in first step ,any pointers are appreciated .