ModelBinder does not update the session key

Question

I created a Website(it is poco object) model binder, that checks the sessions: public class WebsitesModelBinder:IModelBinder { private const string websitesSessionName = "SelectedSite";

    #region IModelBinder Members

    public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        if (bindingContext.Model != null)
            throw new InvalidOperationException("Invalid");

        Website site = (Website)controllerContext.HttpContext.Session[websitesSessionName];
        if (site == null)
        {
            site = new Website();
            controllerContext.HttpContext.Session[websitesSessionName] = site;
        }
        return site;
    }

    #endregion
}

In the global.asax file I registered the model binder for typeof website. In my controller action, the action gets the website as a parameters and updates it such as: public ActionResult Websites(Website SelectedSite) {
var sites = db.Websites.ToList(); if (SelectedSite.ID == 0) SelectedSite = sites[0]; ViewData["Selectedsite"] = SelectedSite;

        return View(sites);
    }

However model binder never updates the session Any ideas?