Firing Events From Future Instances?

I have a class, A, that contains an object of a class B. The B class can fire an event that is handled by A.

public class B
{
    public Event EventHandler SomethingHappened;
}

public class A
{
    private B _b;

    public class A(B b)
    {
        _b = b;
        _b.SomethingHappened += OnSomethingHappened();
    }

    public void OnSomethingHappened()
    {
    }
}

I would like to test all of this, and I would like to test fire the event in B so that my faked instance of A can process it:

var fakeB = Isolate.Fake.Instance<B>(Members.CallOriginal);
Isolate.Swap.AllInstances<B>().With(fakeB);

var fakeA = Isolate.Fake.Instance<A>(Members.CallOriginal, ConstructorWillBe.Called, fakeB);
Isolate.Swap.AllInstances<A>().With(fakeA);

Isolate.Invoke.Event(() => fakeB.SomethingHappened += null, /*event parameter*/);

This doesn't seem to work. If I put a breakpoint at A.OnSomethingHappened(), it is never hit. Any ideas as to why?