How to add and retrieve items to and from mock repository with Rhino Mocks

Why does this test fail? How do you add and retrieve items from a mock repository?

[TestMethod]
public void Can_Mock_Sales_Force_Repo()
{
    // arrange
    var salesForcePolicyRepo = MockRepository.GenerateMock<IRepository<SalesForcePolicy>>();

    // act
    var salesForcePolicy = new SalesForcePolicy { PolicyNumber = "123456" };
    IEnumerable<SalesForcePolicy> salesForcePolicies = new List<SalesForcePolicy> { salesForcePolicy };
    salesForcePolicyRepo.Stub(x => x.FindByExp(null)).IgnoreArguments().Return(salesForcePolicies.AsQueryable());

    // assert
    Assert.IsTrue(salesForcePolicyRepo.Contains(salesForcePolicy));
}

The FindByExp method is in the base interface.

public interface IRepository<T> : IReadOnlyRepository<T> where T : class
{
    void EnrollInUnitOfWork(IUnitOfWork unitOfWork);
    IQueryable<T> FindBy(Func<T, bool> predicate);
    IQueryable<T> FindBy(Func<T, bool> predicate, params Expression<Func<T, object>>[] includes);
    void Add(T item);
    void Add(IEnumerable<T> items);
    void Remove(T item);
    void Remove(IEnumerable<T> items);
    void Update(T item);
    int Execute(string command, params object[] parameters);
    IEnumerable<T> ExecWithStoreProcedure(string query, params object[] parameters);
    void Attach(T item);
}

public interface IReadOnlyRepository<T> where T : class
{
    int Count { get; }
    bool Contains(T item);
    IQueryable<T> FindAll();
    IQueryable<T> FindAll(params Expression<Func<T, object>>[] includes);
    IQueryable<T> FindByExp(Expression<Func<T, bool>> predicate);
    IQueryable<T> FindByExp(Expression<Func<T, bool>> predicate, params Expression<Func<T, object>>[] includes);
}