Mocking a protected field in a base class?

Is it possible to mock a protected field in a base class when testing a derived method?

Here's a sample. I want to test Foo() method, but wish to ingore the calls to the protected field "log". Currently it throws an null exception.

I'm guess that the base class isnt created like normal when mocking? I cant find much documentation on how class hierarchies are handled.

   public interface ILog
    {
        void Debug(string s);
    }

    public class Logger : ILog
    {
        public void Debug(string s)
        {
        }
    }

    public abstract class AbstractBaseClass
    {
        protected ILog log = new Logger();
    }

    public class BaseClassA : AbstractBaseClass
    {        
    }

    public class BaseClassB : BaseClassA
    {
        protected string Foo()
        {
            log.Debug("test"); // How do I mock this call??

            return "abc";
        }        
    }

    [TestClass]
    public class UnitTest3
    {
        [TestMethod]
        public void TestMethod1()
        {
            var mockB = Isolate.Fake.Instance<BaseClassB>();
            Isolate.NonPublic.WhenCalled(mockB, "Foo").CallOriginal();

            string s = Isolate.Invoke.Method(mockB, "Foo") as string;

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