NullReferenceException with example code

    public interface ISearch2
    {
        string GetString(int type);

        string GetFromGuid(Guid g);
    }

    public interface IProvision
    {
        ISearch2 Search { get; }
    }

    public abstract class BaseClass2
    {
        protected string GetFromGuid(IProvision p, Guid g)
        {
            return p.Search.GetFromGuid(g);
        }
    }

    [TestClass]
    public class UnitTest2
    {
        [TestMethod]
        public void UnderTest_Scenerio_Behaviour()
        {
            var mockInterface = Isolate.Fake.Instance<IProvision>();

            Guid myGuid = new Guid("{817B3757-F836-478B-B327-77501F8182EA}");
            Guid myGuid2 = new Guid("{8B4964BB-B6D8-4930-B38F-C32482E1FFBD}");

            Isolate.WhenCalled(() => mockInterface.Search.GetFromGuid(myGuid)).WithExactArguments().WillReturn("one");
            Isolate.WhenCalled(() => mockInterface.Search.GetFromGuid(myGuid2)).WithExactArguments().WillReturn("two"); //XXX this throws a null reference!!

            var mockBase = Isolate.Fake.Instance<BaseClass2>();
            Isolate.NonPublic.WhenCalled(mockBase, "GetFromGuid").CallOriginal();

            string s = Isolate.Invoke.Method(mockBase, "GetFromGuid", mockInterface, myGuid) as string;

            Isolate.Verify.WasCalledWithExactArguments(() => mockInterface.Search.GetFromGuid(myGuid));

            Assert.IsNotNull(s);
            Assert.AreEqual(s, "one");
        }