自己开发的服务器控件发生设计时的错误,请指教.

当添加items后,在Design和Source切换发生的
"x:control" could not be set on property "Items"
----------------------
 [PersistenceMode(PersistenceMode.InnerDefaultProperty)]     [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        public List<ListItem> Items
        {
            get
            {
                EnsureChildControls();
                if (_items == null)
                {
                    _items = new List<ListItem>();
                }
                return _items;
            }
            set 
            {
                if (!DesignMode)
                {
                    _items = value;
                    JavaScriptSerializer seri = new JavaScriptSerializer();
                    string output = seri.Serialize(Items);
                    _extender.ListItem = output;                }
            }
           
        }
-------------------------- [Serializable()]
    public class ListItem
    {
        private string _name;
        private string _value;        public string Name
        {
            get { return _name; }
            set { _name = value; }
        }
    
        public string Value
        {
            get { return _value; }
            set { _value = value; }
        }
    }
-----------------------只是设计时报这样的错误,控件正常运行.