收藏 不显示删除回复显示所有回复显示星级回复显示得分回复 帮忙查错啊：s:combobox与list的问题

错误org.apache.jasper.JasperException: tag 'combobox', field 'list', name 'questionID': You must specify a collection/array/map/enumeration/iterator. Example: people or people.{name} - [unknown location】
 JSP页面代码：
    <s:property value="tip"/>
    <s:form action="question.action">
        <s:combobox list="#requestScope.QuestionTypeList"
        label="问题类型"
        listKey="key"
        listValue="value"
        name="questionID"
         >
        </s:combobox>
        <s:textfield name="questionKye" label="关键字"></s:textfield>
        <s:textarea name="content" cols="10" rows="5"></s:textarea>
        <s:submit value="提问问题"></s:submit> 
    </s:form> 
    action代码：
    public class QuestionAction extends ActionSupport{
    private String content;
    private String questionKye;
    private Integer questionID;
    
    private QuestionDao dao = new QuestionDaoImpl();
    
    

    public String add() throws Exception {
    return SUCCESS;
} public String prepare() throws Exception {
    List list =dao.listQuestionType();
Map map = ActionContext.getContext().getSession();
   map.put("QuestionTypeList", list);
return INPUT;
}

public String getContent() {
return content;
}
public void setContent(String content) {
this.content = content;
}
public String getQuestionKye() {
return questionKye;
}
public void setQuestionKye(String questionKye) {
this.questionKye = questionKye;
}
public Integer getQuestionID() {
return questionID;
}
public void setQuestionID(Integer questionID) {
this.questionID = questionID;
}
}
   struts.xml代码
    
      <action name="question" class="com.amaker.action.QuestionAction">
          <result name="input">/Question.jsp</result>  
      </action>  
  跪求各位大虾帮帮忙 小弟在此感激不尽