object reference not set

i have following code in page load

conn = new SqlConnection();             conn.ConnectionString = Session["Constr"].ToString();             if (Session["Constr"] == null)             {                 Response.Redirect("index.ASPX", false);             }

if i navigaet  from index page i get the pages properly

but if i directly type in the test.aspx in the url instead of navigation from index i get above error inspite of checking for is null