WCF REST POST of JSON: Parameter is empty

Using Fiddler I post a JSON message to my WCF service. The service uses System.ServiceModel.Activation.WebServiceHostFactory

[OperationContract]
[WebInvoke
(UriTemplate = "/authenticate",
       Method = "POST",
       ResponseFormat = WebMessageFormat.Json,
       BodyStyle = WebMessageBodyStyle.WrappedRequest
       )]
String Authorise(String usernamePasswordJson);

When the POST is made, I am able to break into the code, but the parameter usernamePasswordJson is null. Why is this?

Note: Strangly when I set the BodyStyle to Bare, the post doesn't even get to the code for me to debug.

Here's the Fiddler Screen: