Session Is NULL When Running .NET MVC Inside Of .NET

If I create an ASP.NET Web Application project and then add an ASP.NET MVC 2 to it using the default routes defined like so

routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { action = "Index", id = UrlParameter.Optional }
);

The Session object is NULL when I try and access it in the action methods of the controllers. If I change my routes to this.

routes.MapRoute(
"Default", // Route name
"{controller}.aspx/{action}/{id}", // URL with parameters
new { action = "Index", id = UrlParameter.Optional }
);

Everything works just fine. For whatever reason having the .aspx extension allows for session to be used, but the later doesn't. I'm using .NET 3.5 for everything.