unit testing a webapi async action

I am new to unit testing but trying to get my head around it to try improve the quality of code I write. I have created a webapi2 project that returns a customer like ``` public async Task<IHttpActionResult> GetCustomer([FromUri]int id) { var customer = await _repository.FindCustomerAsync(id); return Ok(customer); } ``` My repository ``` public async Task<Customer> FindCustomerAsync(int id) { using (var context = new MyContext()) { var result = await context.Customers.FindAsync(id); return result; } } ``` Previously I was not returning an async task at it was very easy to test. Migrating the action to a async task has made it a little hard for me to test. I am using Moq and Xunit and my attempt at a unit test looks like ``` [Fact()] public async void GetCustomer() { var id = 2; _customerMock.Setup(x => x.FindCustomerAsync(id)) .Returns(Task.FromResult(FakeCustomers() .SingleOrDefault(cust => cust.customerID == id))); var controller = new CustomersController(_customerMock.Object).GetCustomer(id); var result = await controller as Customer; Assert.NotNull(result); //Assert.IsType<OkNegotiatedContentResult<Customer>>(negotiatedResult); //Assert.Equal(negotiatedResult.Content.customerID, id); } ``` My FakeCustomers ``` private IQueryable<Customer> FakeCustomers() { return new List<Customer>() { new Customer() { customerID = 1, firstName = "Brian", lastName = "Smith" }, new Customer() { customerID = 2, firstName = "Tom", } }.AsQueryable(); } ``` the test always fails when trying to cast to Customer {"Object reference not set to an instance of an object."} What I am doing wrong with my test?