Testing ActionResult<T> when BadRequest() is returned

I am trying to build a unit test around a controller using .NET Core's Web API. I'm running into an issue when creating a test for a negative scenario where the response should be a BadRequest(string):

ThingController.cs

[HttpPost("thing/{id}")]
[ProducesResponseType(400)]
public async Task<ActionResult<Thing>> Add(string id, [FromBody] Thing thing)
{
  if (String.IsNullOrEmpty(id))
  {
    return BadRequest("Must provide Thing ID");
  }

  return this.thingRepo.AddThing(id, thing);
}

ThingControllerTests.cs

[Test]
public async Task AddThing_FailOnEmptyId() {
  var mockRepo = new Mock<IThingRepository>();
  var testController = new ThingController(mockRepo.Object);
  var result = await testController.AddThing("", this.validThing);

  Assert.That(result, Is.InstanceOf<BadRequestObjectResult>());
}

In this case, the value of result is always Thing where I expect it to be BadRequestObjectResult.

Some additional details:

• I've verified that the test exercises the correct branch and performs return BadRequest("Must provide Thing ID") like I expected.
• If I change the return type from Task<ActionResult<T>> to Task<ActionResult> the test passes like I expect.
• After reading the docs on the Microsoft site Controller action return types - ActionResult T type I think I have the code itself correct enough for this test.

Is there a specific cast I need to perform to get the proper type? Am I missing a detail about NUnit that will make this work?