请教各位

我这段代码总是报：未知的运行错误
代码如下:  function check_pid()
{

var pid=document.all("pid").value;
var url = "select_practice.aspx?pid="+pid+"&s="+Math.random();
var xmlDoc = new ActiveXObject("Msxml2.DOMDocument");
var xmlServerHttp = new ActiveXObject("Msxml2.XMLHTTP");
var xmlStr = "";
try 
{
xmlDoc.async = false;
xmlServerHttp.open("GET", url, false);
xmlServerHttp.send();
document.getElementById("pid2").innerHTML= xmlServerHttp.responseText;
}
catch(e)
{
  alert(e.message);
} }
后端代码如下：            if (pid == "")
            {
                Response.Write("<script>alert('请输入序号');</script>");
            }
            else
            {
                StringBuilder html = new StringBuilder();
                if (pid =="1")
                {
                    html.Append("<td width='6%'>A</td>");
                    Response.Write(html.ToString());
                    html = null;
                }
            }