How do you fool a ServiceStack ISession with Moq and StructureMap?

I am using ServiceStack / StructureMap / Moq. The service makes a call to the session, which is a type of ServiceStack.CacheAccess.ISession. For unit tests, I created a Mock object using Moq and added it to the StructureMap configuration:

protected Mock<ISession> sessionMock = new Mock<ISession>();
ObjectFactory.Configure(
    cfg =>
       {
           cfg.For<ISession>().Use(sessionMock.Object);

However, I was not surprised when the Session object was null - I am sure that I am going one step further. What else do I need to do to populate my Session property with a mock object?

[EDIT] Here's a simple test case

Code to verify. Simple inquiry / service

[Route("getKey/{key}")]
public class MyRequest:IReturn<string>
{
    public string Key { get; set; }
}

public class MyService:Service
{
    public string Get(MyRequest request)
    {
        return (string) Session[request.Key];
    }
}

Base Test Class and MockSession Classes

// test base class
public abstract class MyTestBase : TestBase
{
    protected IRestClient Client { get; set; }

    protected override void Configure(Container container)
    {
        // this code is never reached under any of my scenarios below
        container.Adapter = new StructureMapContainerAdapter();
        ObjectFactory.Initialize(
            cfg =>
                {
                    cfg.For<ISession>().Singleton().Use<MockSession>();
                });
    }
}

public class MockSession : ISession
{
    private Dictionary<string, object> m_SessionStorage = new Dictionary<string, object>();

    public void Set<T>(string key, T value)
    {
        m_SessionStorage[key] = value;
    }

    public T Get<T>(string key)
    {
        return (T)m_SessionStorage[key];
    }

    public object this[string key]
    {
        get { return m_SessionStorage[key]; }
        set { m_SessionStorage[key] = value; }
    }
}

And tests. See Comments for where I see failure. I did not expect versions 1 and 2 to work, but I hope that version 3 will be.

[TestFixture]
public class When_getting_a_session_value:MyTestBase
{
    [Test]
    public void Test_version_1()
    {
        var session = ObjectFactory.GetInstance<MockSession>();
        session["key1"] = "Test";
        var request = new MyRequest {Key = "key1"};
        var client = new MyService();  // generally works fine, except for things like Session
        var result = client.Get(request);  // throws NRE inside MyService
        result.ShouldEqual("Test");
    }

    [Test]
    public void Test_version_2()
    {
        var session = ObjectFactory.GetInstance<MockSession>();
        session["key1"] = "Test";
        var request = new MyRequest {Key = "key1"};
        var client = ObjectFactory.GetInstance<MyService>();
        var result = client.Get(request);  // throws NRE inside MyService
        result.ShouldEqual("Test");
    }
    [Test]
    public void Test_version_3()
    {
        var session = ObjectFactory.GetInstance<MockSession>();
        session["key1"] = "Test";
        var request = new MyRequest {Key = "key1"};
        var client = CreateNewRestClient();
        var result = client.Get(request);  // throws NotImplementedException here
        result.ShouldEqual("Test");
    }
}