为何ajax调用form中元素会出错

<form id="form1" runat="server">
<table>
<tr>
<td id="test"></td>
</tr>
</table>
</form>         var req = null;
         function processReqChange()
         {
          
            if(req.readyState == 4 && req.status == 200)
            {
                 var dobj = document.getElementById("test");
                dobj.innerHTML = req.responseText;
            }
            else
            {
                var dobj2 = document.getElementById( "test" );
                dobj2.innerHTML ="test";
            }
         }
         
         function getResponse(url)
         {
          if(window.XMLHttpRequest) {
          try { req = new XMLHttpRequest();
          } catch(e) { req = false; }
        } else if(window.ActiveXObject) {
         try { req = new ActiveXObject('Msxml2.XMLHTTP');
         } catch(e) {
        try { req = new ActiveXObject('Microsoft.XMLHTTP');
        } catch(e) { req = false; }
        } 
        }
       if(req) {
       req.onreadystatechange = processReqChange;
       req.open('GET', url,true);
       
       req.send('');
       }
     }返回req.responseText出错
但将上面的table放在form之外就可以了，请问如何解决这个问题，我页面有很多服务器端控件，所以需要form