求助，程序调试

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _add : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void Button2_Click(object sender, EventArgs e)
    {
        int textNum;
        int btnNum;
         textNum = Convert.ToInt32(this.TextBox1.Text);
         btnNum = Convert.ToInt32(this.TextBox2.Text);
        for (int i=0;i<textNum;i++)
        {
            this.Panel1.Controls.Add(new TextBox());
        }
        for (int j = 0; j <btnNum; j++)
        {
            this.Panel1.Controls.Add(new Button());
        }

    }
}

为什么这句textNum = Convert.ToInt32(this.TextBox1.Text);老是说：输入字符串的格式不正确。
它和这句是一样的btnNum = Convert.ToInt32(this.TextBox2.Text);，为什么会出现这样的结果？