ASP.NET shopping cart error in C#?

i am working on a shopping cart for a project and i'm really stuck. ASP.NET seams to be really hard :(
I get this error
Unable to cast object of type 'System.String' to type 'System.Collections.Generic.List`1[System.String]'.
At this line of code
var values = (List<string>)Session["Cart"];
Any help would be nice!
if (Session["Cart"] == null)
{
Response.Redirect("Products.aspx", false);
}

{
var values = (List<string>)Session["Cart"];
if (values.Count <= 0)
{
Response.Redirect("Products.aspx", false);
}
}
This error only happens if i have 1 item only in the cart