Problem in implementing session..

Object Reference not set to Instance of an object Exception

// Checkout.aspx

Order o = new Order();
        o.CustomerID = User.Identity.Name;
        Session["CustomerID"] = o.CustomerID;
        o.OrderDate = DateTime.Now;
        o.FirstName = firstnametxt.Text;
        Session["FirstName"] = o.FirstName;
        o.LastName = lastnametxt.Text;
        Session["LastName"] = o.LastName;
        o.Company = companytxt.Text;
        Session["Company"] = o.Company;
        o.Address = addresstxt.Text;
        Session["Address"] = o.Address;
        o.Country = dpcountry.Text;
        Session["Country"] = o.Country;
        o.City = citytxt.Text;
        Session["City"] = o.City;
        o.Province = provincetxt.Text;
        Session["Province"] = o.Province;
        o.ZipCode = zipcodetxt.Text;
        Session["ZipCode"] = o.ZipCode;
        o.Telephone = telephonetxt.Text;
        Session["Telephone"] = o.Telephone;
        o.Fax = faxtxt.Text;
        Session["Fax"] = o.Fax;

protected void Page_Load(object sender, EventArgs e)
    {
 lblfirstname.Text = Session["FirstName"].ToString();
        
        lbllastname.Text = Session["LastName"].ToString();
        lbladdress.Text = Session["Address"].ToString();
        lblphoneno.Text = Session["Telephone"].ToString();
        lblprovince.Text = Session["Province"].ToString();
        lblcity.Text = Session["City"].ToString();
        lblpostalcode.Text = Session["ZipCode"].ToString();
        bind();

    }