Tailspin spyworks my Shopping cart SESSION DOES NOT EXIST

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using SkWorks;
using System.Web.UI.WebControls;
using System.Web.Security;
using CommerceModel;
using System.Data;
using System.Diagnostics;

//--------------------------------------------------------------------------------------------------------------------------------------------------+
// TODO: In a future version of this application we will seperate this logic into a seperate Business Objets Layer
//--------------------------------------------------------------------------------------------------------------------------------------------------+
namespace SkWorks
{
    public struct ShoppingCartUpdates
    {
        public int ProductId;
        public int PurchaseQuantity;
        public bool RemoveItem;
    }


    public partial class MyShoppingCart
    {
        public const string CartId = "SkWorks_CartID";

        //------------------------------------------------------------------------------------------------------------------------------------------+
        public String GetShoppingCartId()
        {
            string str = HttpContext.Current.Session["CartId"].ToString();
            Session[CartId] = System.Web.HttpContext.Current.Request.IsAuthenticated ? User.Identity.Name : Guid.NewGuid().ToString();
            if (Session[CartId] == null)
            {

            }
            return Session[CartId].ToString();
        }