Unit Testing IHttpActionResult Controller - Testing for 200 OK fails

This is probably a fairly simple question but I'll admit, I'm stuck.

I have a controller that is returning IHttpActionResult and I need to write unit tests for this.

Here's the controller:

public IHttpActionResult GetPerson(int id) {

    Person person = repository.Get(id);

    if (person == null) {
        throw new HttpResponseException(HttpStatusCode.NotFound);
    }

    return Ok(new {
        User = person
    });

}

Here's the unit test:

[TestMethod]
public void GetReturnsValidPerson() {

    var userController = new UserController();

    IHttpActionResult actionResult = userController.GetPerson(1);

    Assert.IsInstanceOfType(actionResult, typeof(OkResult));

}

And here's the error from the test:

Assert.IsInstanceOfType failed. Expected type:. Actual type:.

What exactly is going on here? The return from the controller is an HTTP 200 Ok response. Why is this expecting OkNegotiatedContentResult?