Why does my inbound Response Message contain the wrapper psnonxml?

I have a REST Web Service that uses a Document Type Message for a GET Method Service Operation.  This service operation is triggered using an app engine.    The app engine is failing with the error message, ‘XML parser error ParseXmlString’.  The service operation goes to DONE status and does not contain any error or warning message.

The Response Message in the gateway’s msgLog.html contained the following.  I can copy this into Notepad++ and format using a JSON Format Plugin without any issue.

The Response Message in the details of the Service operation contained the following. 

When I copy the above into NotePad ++ and use the Jason Formatter the following message pops up.