Restlet POSTing JSON to Appengine error

Question

We have a small app with Restlet on the GAE server and GWT and Android clients. Restlet serves GWT-serialized replies to GWT clients and JSON to Android clients.

All is ok with GWT-serialization both locally and on AppEngine production servers.

All is ok with Android (JSON) clients talking to local dev server.

Android taking to production AppEngine server GETs JSON replies, but POST fails. There is no error or warning in logs. The function on server gets called but parameter passed is null.

This is the offending code:

@Post("json")
public void createLocationJSON(Location location) {  // location is always null
    // do something with 'location'
}