Mocking Instance Variable in Subclass

I have a class

public class C1 : A1<I1, I2<I1>, I3<Doc>, I4<I1>, C2>

A1 is defined as

public abstract class A1<TA, TB, TC, TD, TE> : A1<TA, TE>

The class it inherits from is defined thus

public abstract class A1<TA, TE> : A2<TA, TE>

A2 is defined like this

public abstract class A2<TA, TE> : A3<TA>
    {
        protected readonly Foo<TA, TE> Foo;

        protected A2(Foo<TA, TE> foo)
        {
            Foo = foo;
        }

I am testing C1 using Rhino Mocks and I want to mock a method Bar on Foo. I am having trouble doing this.

I have tried, this but it does not work.

var mock = MockRepository.GenerateMock<A2<I1, C2>>();
mock.Stub(m => m.Bar())
                .Return("output");

What am I doing wrong?