I'm having an issue where my session is expiring, 

```
Session["UserId"] = userId;

```

but the authentication cookie is still there so the [authorize] attribute is still allowing the user to navigate the system until the system tries to read the session variables and errors out. Null exception

Are there any thoughts on how to make the auth cookie go away when the session expires? I would certainly appreciate any insight as I am new to ASP.NET MVC 3.