JSONP: Overlays with Optional Attributes (HostedModeException's), How?

I'm attempting to perform JSONP (cross domain ajax), just like in this great article (thanks) http://eggsylife.co.uk/2010/04/22/gwt-2-jsonp-and-javascript-overlays-with-jsonprequestbuilder/

The json response has optional attributes, it could be...

    {error: "yay"}

or

    {response: 53}

The optional attributes are causing my JavaScript Overlay's + JSNI grief... If I get don't get  {error: "yay"} and I invoke getResponse()....

    public final native int getResponse()/*-{

    return this.response;

    }-*/;

it throws...

com.google.gwt.dev.shell.HostedModeException: Something other than a double was returned from JSNI method '@com.acme.client.Blah::getResponse()': JS value of type undefined, expected int

Summary + Questions....

1. The JSONP response is doing nothing wrong - it's is perfectly valid response. Some attributes are optional and Overlays seem to deals with them poorly.

2. According to the GWT doco,  these HostedModeException only occur in Dev mode, and never thrown in production mode. So what happens when running in Production Mode?

3. Are there alternatives to wrapping the JSNI+JSON objects? Can I do something in JSNI that will handle the undefined attribute happily?

Advice welcome, thanks for reading :)

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