Static Method's Behaviour once set is not possible to alter

public class SampleClass
    {
        public int Sum(int a, int b)
        {
            return _Sum(a, b);
        }

        private static int _Sum(int a, int b)
        {
            return a + b;
        }
    }

/// <summary>
    /// Summary description for SampleClassTest
    /// </summary>
    [TestClass]
    public class SampleClassTest
    {
        [TestMethod]
        public void TestSum()
        {
            SampleClass fakeSampleClass = Isolate.Fake.Instance<SampleClass>(Members.CallOriginal);
            Isolate.NonPublic.WhenCalled(typeof(SampleClass), "_Sum").WillReturn(5);
            int result = fakeSampleClass.Sum(2, 3);
            Assert.AreEqual(result, 5);
        }

        [TestMethod]
        public void TestSumPrivate()
        {
            var result = Isolate.Invoke.Method(typeof(SampleClass), "_Sum", 5, 5);
            Assert.AreEqual(result, 10);
        }
    }