This is what I was seeing in Postman...
However when I switched to raw output, I could see it was displaying 'Not Found'. That was an error string that the service could return. So what was going on here?
Well the service was annotated with this:
That meant that Postman was expecting a JSON object response and was getting a simple string instead!
So Postman was correctly saying that the service response was returning an unexpected value based on the application specification (via the @Produces annotation).