Struts2 optiontransferselect标签老提示错误。。。。

<%@ page language="java" pageEncoding="utf-8"%>
<%@ taglib uri="/WEB-INF/struts-tags.tld" prefix="s"%><html>
<title>
</title><body>
<form><s:optiontransferselect
            tooltip="Select Your Favourite Cartoon Characters"
            label="Favourite Cartoons Characters"
            name="leftSideCartoonCharacters" 
            leftTitle="Left Title"
            rightTitle="Right Title"
            list="{'Popeye', 'He-Man', 'Spiderman'}" 
            multiple="true"
            headerKey="headerKey"
            headerValue="--- Please Select ---"
            emptyOption="true"
            doubleList="{'Superman', 'Mickey Mouse', 'Donald Duck'}" 
            doubleName="rightSideCartoonCharacters"
            doubleHeaderKey="doubleHeaderKey"
            doubleHeaderValue="--- Please Select ---" 
            doubleEmptyOption="true"
            doubleMultiple="true" />
</form></body>
</html>我按照标准例子拷上怎么也无法让数据从左栏跑到右栏去，老提示找不到对象，是不是什么东西没引用啊