请教struts+ajax出现TypeError: form.elements[fieldName] is undefined

请问我在用strut2+dwr验证时，能够进行验证，但是比如我在对文本框输入空时就会验证，但是同时在firefox里弹出TypeError: form.elements[fieldName] is undefined对话框，这是怎么回事呢？？<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<%@ taglib prefix="s" uri="/struts-tags" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <base href="<%=basePath%>">
    
    <title>My JSP 'user_register.jsp' starting page</title>
    
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">    
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
     <s:head theme="ajax"/>
  </head>
  
  <body>
   <s:form  id="form1" action="userLogin" method="post" theme="ajax" validate="true">
    <s:textfield label="员工编号" name="user_num" id="user_num"></s:textfield>
    <br>
    <s:password label="密码" name="user_password" id="user_password"/>
    <s:submit value="登录"/>
   </s:form>
   
  </body>
</html>