problem with using datatable to insert row

[ykyk]

when i first add an item, gridview can show the data from dtcart. when i add second item, the error "Input array is longer than the number of columns in this table." appear in ##. Did i use data table wrongly?

protected void addToCartbtn_Click(object sender, EventArgs e)
        {
            int quantity = Convert.ToInt32(tb_qty.Text);
            double unitPrice = Convert.ToDouble(pPrice.Text);
            double subTotal = quantity*unitPrice;
            

            DataTable dtCart = new DataTable();

            if (Session["Cart"] == null)
            {
                dtCart.Columns.Add("productID");
                dtCart.Columns.Add("productName");
                dtCart.Columns.Add("unitPrice");
                dtCart.Columns.Add("quantity");
                dtCart.Columns.Add("subTotal");     
            }
            else
            {
                Session["Cart"] = dtCart;
            }
            
            DataTable dtprod = new DataTable();
            dtprod = action.getproductDetails(Request.QueryString["productID"]);
            
            bool check = false;

            for (int i = 0; i < dtCart.Rows.Count; i++)
            {
                if (dtCart.Rows[i][0].ToString() == Request.QueryString["productID"].ToString())
                {
                    check = true;
                    Response.Redirect("FrontShoppingCart.aspx");
                }
                else
                {
                    dtCart.Rows.Add(new object[] {Request.QueryString["productID"],
                                                  dtprod.Rows[0]["productName"].ToString(),
                                                  dtprod.Rows[0]["productPrice"].ToString(),
                                                  quantity,
                                                  subTotal
                                                 });
                }
            }
            
            if (check == false)
            {
                   ## dtCart.Rows.Add(new object[] {Request.QueryString["productID"],
                                                    dtprod.Rows[0]["productName"].ToString(),
                                                    dtprod.Rows[0]["productPrice"].ToString(),
                                                    quantity,
                                                    subTotal});
             }

                    Session.Add("Cart", dtCart);
}