struts2 select 驗證異常

今天用到s:select 然後進行表單驗證，老出錯，

tag 'select', field 'list', name xxxx: The requested list key '' could not be resolved as a collection/array/map/enumeration/iterator type

原來是驗證失敗返回input頁面時，由於沒有取到list的值，所以報上述異常