我的<s:fielderror>標簽出錯

<%@?page?language="java"?contentType="text/html;?charset=UTF-8"
????pageEncoding="UTF-8"%>
<%@taglib?prefix="s"?uri="/struts-tags"?%>
<!DOCTYPE?html?PUBLIC?"-//W3C//DTD?HTML?4.01?Transitional//EN"?"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=UTF-8">
<title>Insert?title?here</title>
</head>
<body>
????<form?action="LoginAction"?method="post">
????????用戶名：<input?type="text"?name="username"?value="v3"><s:fielderror?name="username"></s>
????????密碼：<input?type="password"?name="password"?value="loveDayDay">
????????書籍1：<input?type="text"?name="bookList[0].username"?value="Java從入門到放棄">
????????書籍2：<input?type="text"?name="bookList[1].username"?value="Struts2從入門到發瘋">
????????年齡：<input?type="text"?name="age"?value="10">
????????<input?type="submit"?value="提交"?/>
????</form>
</body>
</html>