NullReferenceException Accessing Property of MockAll<> Mock

The code I am trying to test has this structure:

public class A
{
    public string S
    {
    }
}

public class B
{
    private A _a;

    public B()
    {
        _a = new A();
        _a.S = "Some String"; 
    }
}

Since the instance of B creates an instance of A internally, I need to make sure the internal instance is mocked as well. Therefore, I have written a unit test that looks something like this:

MockManager.MockAll<A>();
var fakeB = Isolate.Fake.Instance<B>(Members.CallOriginal);

This crashes, because I get a NullReferenceException at the point where B tries to set the S property of A. It says that S is null. Why is that? What am I doing wrong?