parameter ?50 not found in collection

Hi,
      My problem is that whenever i am run my program it give error i.e" parameter ?50 not found in collection". code is below. pese help
public partial class AddUser : System.Web.UI.Page
{
Genral ObjGen = new Genral();
private string strCon = ConfigurationManager.ConnectionStrings["MyCon"].ToString();
protected void Page_Load(object sender, EventArgs e)
{
btn_SUBMIT.Attributes.Add("onclick", "return CheckMandatory();");
if (Session["User"] == null)
{
Session.Abandon();
Response.Redirect("Login.aspx");
}
else if (Session["DESIG"].ToString() != "RESELLER")
{
Response.Redirect("Instructions.aspx");
}
else if (Session["DESIG"].ToString() == "RESELLER" && long.Parse(Session["Balance"].ToString()) == 0)
{
Page.ClientScript.RegisterStartupScript(this.GetType(), "Check", "<script language='javascript'>" + "\n" + "alert('you balance is i ...