Cannot mock public members

Hi, I have two classes

    public class ClassSim
    {
        public void DoNothing()
        {

        }

        public ClassSim()
        {

        }
    }

    public class ClassComp
    {
        public ClassSim sim=new ClassSim();
        public ClassComp()
        {
            sim=new ClassSim();
        }
        public void CallClassSim()
        {
            //sim = new ClassSim();
            sim.DoNothing();
        }
    }

I found that this test

        [Test]
        [Isolated]
        public void TestNest()
        {
            ClassComp compFake = Isolate.Fake.Instance<ClassComp>(Members.CallOriginal);
            ClassSim simFake = Isolate.Fake.Instance<ClassSim>();
            Isolate.WhenCalled(()=>compFake.sim).WillReturn(simFake);

        }

will return an exception. I don't think this is the valid behavior because if I change sim from variable type to a property type, then the test will run just fine :D
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Mercedes-Benz W221 specifications