Null Value exception for testResuts param when importing test results using REST API

Hi,

I have a script written to import test results into VS Team Services online using the REST API. I can query the API fine, and can successfully create a Test Run using the API, but I'm struggling to get this to work. I have Test cases listed within the TEST module in VS Online, and I'm trying to set the run state and outcome of one of the test cases by id, but when I do so the API returns a 400 error with message:

Value cannot be null.\r\nParameter name: testResults

Any ideas what I'm missing here? The examples in the API documentation appear much less complete than this. The run (id=56) exists and is In Progress and the test case is set as Ready (though it matters not which test id I choose). I've replicated the call and the JSON payload in the HttpRequester plugin for Firefox - screenshot as follows to show what I'm doing:

Any help much appreciated...