Cannot create iterator for this collection

我要实现的功能是：用bean：write标签显示数据库中的内容。代码如下：
public class ShowmessageAction extends Action {
private AdminDAO adminDAO;

public AdminDAO getAdminDAO(){
return adminDAO;
}
public void setAdminDAO(AdminDAO adminDAO){
this.adminDAO=adminDAO;
}
public ActionForward execute(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {
ShowmessageForm showmessageForm = (ShowmessageForm) form;// TODO Auto-generated method stub
return null;
}
public ActionForward list(ActionMapping mapping, ActionForm form,
HttpServletRequest request, HttpServletResponse response) {
List list = adminDAO.findAll();
request.setAttribute("show",list);
System.out.println(list);    
return mapping.findForward("showmessage");
}
}<%@ page language="java" pageEncoding="GBK"%>
<%@ taglib uri="http://struts.apache.org/tags-bean" prefix="bean"%> 
<%@ taglib uri="http://struts.apache.org/tags-html" prefix="html"%>
<%@ taglib uri="http://struts.apache.org/tags-logic" prefix="logic"%><html> 
<head>
<title>JSP for ShowmessageForm form</title>
</head>
<body>
<html:form action="/showmessage">
<h2 align="center"><font color="blue">Admin</font></h2>
    <div align="center"> <table align="center" border="1px" cellspacing="0" cellpadding="10" width="300" height="20">
<tr>
<th>
用户名
</th>
<th>
密码
</th>
</tr>
<logic:iterate id="show" name="showmessageForm">
            <tr bgColor=#E4E8EF>
<td>
<bean:write name="show" property="name"/> 
                    </td>
<td>
<bean:write name="show" property="password"/>
</td>
</tr>
</logic:iterate>
</table>
</div>
</html:form>
</body>
</html>错误提示：javax.servlet.ServletException: javax.servlet.jsp.JspException: Cannot create iterator for this collection
请问为什么会有这个错误，如何解决，谢谢！