private generic method results in NullReferenceException

I am working with TypeMock 5.3.1. If I create a group of classes like this:

   public class A<T>
   {
      virtual public int GetOne()
      {
         return 1;
      }
   }

   public class B<T> : A<T>
   {
      public override int GetOne()
      {
         return GetTwo();
      }

      private int GetTwo()
      {
         return 2;
      }
   }

   public class C : B<int>
   {

   }

And run the code in a test like this:

   [TestMethod]
   public void TestMethod()
   {
      C c = new C();

      Isolate.NonPublic.WhenCalled(c, "GetTwo").DoInstead(callContext =>
      {
         return 20;
      });

      Assert.AreEqual(20, c.GetOne());
   }

I get a NullReferenceException. Is this a bug, or am I doing something wrong?

The example is nonsense, but it mimics facets of some real code that I'm attempting to test. Any help that you can offer would be appreciated.

Thanks!