REST Receiver xPath expression does not work for iDoc

Right now I cannot get the REST Receiver adapter to work like I expect in SAP PO 7.5

The scenario consists out of an outbound iDoc and an inbound REST Message.

When I send out an idoc I keep getting the error below.

Transmitting the message to endpoint <local> using connection IDoc_AAE_http://sap.com/xi/XI/System failed, due to: com.sap.engine.interfaces.messaging.api.exception.MessagingException: com.sap.aii.adapter.rest.ejb.parse.InvalidXPathExpression: Invalid XPath expression "//PERNR"; Message: "Expression did not match."

Here's a screenshot of the REST Receiver adapter configuration:

I do not have the complete xPath above because I was advised not to. But with complete xPath in the configuration is still does not work.

Below is the payload of the incoming iDoc, where field PERNR is available in the last segment of the iDoc.

Any ideas? Thanks in advance.