Test Controller not passed, since controller returns null

I have an API as below:

 private readonly IMyService _myService;
    public MyController(IMyService myService)
    {
        _myService = myService;
    }

    //GET: api/values
    [HttpGet]

    public MyOutput<MyEntity> Get(string f, string o)
    {
        var fItems = JsonConvert.DeserializeObject<Dictionary<string, string>>(f);
        var oItems = GetDictionaryFromStr(o ?? "");

        var myInput = new MyInput<MyEntity>()
        {
            PredicateDictionary = fItems,
            OrderByDictionary = oItems
        };

        var result = _myService.Search(myInput);

        return result;
    }

It works well. Now, I want to write a unit test for my API, using Moq and Xunit;. I want to set the expected values of result, then mock my DIs and call the controller, What I expect is that the return value of controller and my results be equal. but I don't know why result in var result = api.Get(f, o); is null after returns from controller. Is there anything wrong whit my test?

    [Fact]
    public void Should_ReturnResult_When_CallingMyApi()
    {
        //Arrange
        var f = "{'Currency':'UR'}";
        var o = "+Amount";

        var fItems = JsonConvert.DeserializeObject<Dictionary<string, string>>(f);
        var oItems = GetDictionaryFromStr(o ?? "");


        var baseServiseMock = new Mock<IMyService>();
        baseServiseMock 
            .Setup(x => x.Serach(It.Is<MyInput<MyEntity>>
                (i => i.PredicateDictionary== fItems
                 && i.OrderByDictionary == oItems
                )))
            .Returns(new MyOutput<MyEntity>()
            {
                OrderByDictionary = oItems,
                PredicateDictionary = fItems
            });

        var api = new MyController(baseServiseMock.Object);



        //Act
        var result = api.Get(f, o);


        ////Assert
        Assert.Equal(result.PredicateDictionary, fItems);
        Assert.Equal(result.OrderByDictionary, oItems);
    }