moq objects Returns method, should return a null object



I'm developing a Web API, and one of the test I came up with is that, if client makes a GET operation with a Physical Test ID (Physical Test is the resource I'm looking for) and that physical test is not found, the web API should return a 404 status.

Now, I'm using moq framework for my tests and I have the following code:

public void then_if_physical_test_not_found_return_not_found_status()
    var unitOfWork = new Mock<IUnitOfWork>();
    var repository = new Mock<IRepository<PhysicalTest>>();
    repository.Setup(r => r.FindById(It.IsAny<int>())).Returns();
    unitOfWork.Setup(m => m.PhysicalTests).Returns(repository.Object);
    var pt = new PhysicalTestResource(unitOfWork.Object);
    HttpResponseMessage<PhysicalTest> response = pt.GetPhysicalTest(43);
    Assert.AreEqual(HttpStatusCode.NotFound, response.StatusCode)

I need the Returns() method to return a null object, which is going to be what the actual API method would return if the resource is not found.

I tried sending null as a parameter in the Returns() method but had no success.