Mocking the client in Retrofit - callback object is null

Question

I'm following the suggestion mentioned here for mocking retrofit in testing: Square retrofit server mock for testing

It looks like I'm pretty close following this concept except the return object is null. The following is how I construct my mocked vendors response:

public Response getVendorsResponse(String url) {
    String responseString = "";

    if (url.contains("category=one")) {
        responseString = mFixturesMan.use("one_all");
    } else if (url.contains("category=two")) {
        responseString = mFixturesMan.use("two_all");
    }

    return new Response(
            url,
            200,
            "nothing",
            Collections.EMPTY_LIST,
            new TypedByteArray("application/json", responseString.getBytes()));
}

My callback is as follows:

    @Override
    public void success(MyResult result, Response response) {
        // result is null when mocked.
        displayResult(result.getTitle());
    }

When debugging, the response I expect comes through. I see the status and the reason I set.

Does this mean the data being passed as the response is bad? Or could other things be wrong? I've copied (and modified slightly) the text from a real response that I know works, so it's possible it's bad data. I can't seem to find the source of the error though.