Unit Test with Ninject Instantiate

I'm trying to test my service using ninject and an unit test project with visual studio 2012. My inject works ok on my controllers, but when I try to do the same in the unit test class I get an Exception.

System.NullReferenceException: Object reference not set to an instance of an object.

namespace Trex.UnitTests
{
    [TestClass]
    public class GiftServiceTests
    {
        private IGiftService _giftService;

        public void GiftServiceTest(IGiftService giftService)
        {
             _giftService = giftService; 
        }

        [TestMethod]
        public void AddGift()
        {

            var list = _gift.FindAll();  <--- this line throw an exception
        }
    }
}

I think there is something wrong with the injection but I dont get it.