การคำนวณโดยใช้ Session จาก form1 ไป form2 แต่ดันติด Error ที่ฟอม 2 ว่า input string was not in a correct format.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

public partial class Default7 : System.Web.UI.Page
{
    private SqlConnection CN()
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = ConfigurationManager.ConnectionStrings["aaa"].ToString();
        con.Open();
        return con;
    }
    DataSet ds;
    string sql;

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            sql = "select*from tblproduct";
            SqlCommand cm = new SqlCommand(sql, CN());
            ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(sql, CN());
            da.Fill(ds, "product");

            Label1.Text = Session["a"].ToString();
            Label2.Text = null;
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        double price=0;
        double x = Convert.ToDouble(Label1.Text); //ติดErrorตรงนี้ครับ

        if (RadioButtonList1.SelectedIndex == 0)
            price = x;
        else
            price = x * 0.10;

        Label2.Text = (price.ToString("n2" + " บาท"));
    }
}