How to assign a value to a session var in WebForms code behind pages?

Hi,
I'm modifying a web forms app, using c#. I'm struggling with a simple task of assign a value to the session var; then retrieve it for comparison 2 pages later.  Note: I did the same thing many time in MVC, it always worked. MAybe I'm not getting the syntx correct?
Here is the code in my codebehind of the 1st page, where I set a value for the session var.:
protected void Page_Load(object sender, EventArgs e)
{
         Session["PlanSelected"] = "Starter";
         .....
}
2 pages later, I want to retrieve this session var; here is the code in the code behind file of the 4th page:
if ((string)HttpContext.Current.Session["PlanSelected"] == "Starter")    &nb ...