IWS REST/JSON web service throws UnrecognizedPropertyException

I created a proof of concept web service using a RPGLE service program
as the resource.

IWS Config is:

Procedure name:
TESTLITE2
HTTP request method:
PUT
URI path template for method:
*NONE
HTTP response code output parameter:
HTTPSTATUS
HTTP header array output parameter:
HTTPHEADERS
Allowed input media types:
*JSON
Returned output media types:
*JSON


Input parameter mappings:
Unwrapped.
Name: WTHC
Type Struct
Input source: None (The JSON goes in the body of the request)

The JSON is just:
{
"pm_parm1":"one",
"pm_parm2":"two"
}

Which validates ok.

However, when calling the web service from SOAPUI I get the following error:

Error 500: javax.servlet.ServletException:
org.codehaus.jackson.map.exc.UnrecognizedPropertyException:
Unrecognized field "pm_parm1" (Class iseries.wsbeans.testlite2.WTH),
not marked as ignorable at [Source:
com.ibm.ws.webcontainer.srt.SRTInputStream@45584558; line: 2, column:
13] (through reference chain:
iseries.wsbeans.testlite2.WTH["pm_parm1"])

Any suggestions as to the underlying cause of this?

Thanks,

Keith