Stuck whilst trialing. Help?

I'm trialing the product and im also very new to mocking frameworks. Below is a simple example of something I'm trying to acheive in a much bigger project.

I'm trying to write a unit test for the protected method in an abstract class. I was hoping that the below code would inject a mock "Search" interface object that I had arrange to return a know value. But it doesnt!

What have I missed? Note: I actually want the code instead BaseClass.GetString to execute (that's the test) but I want the Search interface it uses to be a mock.

    public interface ISearch
    {
        string GetString(int type);
    }

    public abstract class BaseClass
    {
        public ISearch Search { get; set; }

        protected string GetString(int type)
        {
            return Search.GetString(type);
        }
    }

    [TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestMethod1()
        {
            var mock = Isolate.Fake.Instance<BaseClass>();

            Isolate.WhenCalled(() => mock.Search.GetString(1)).WithExactArguments().WillReturn("one");

            string s = Isolate.Invoke.Method(mock, "GetString", 1) as string;

            Isolate.Verify.WasCalledWithExactArguments(() => mock.Search.GetString(1));
            
            Assert.IsNotNull(s);
            Assert.Equals(s, "one");
        }
    }