Winform DataGrid 的行的Tag有关问题

#region 数据绑定
        /// <summary>
        /// 数据绑定
        /// </summary>
        private void DataGridDataBind()
        {
            gv_SpendType.AutoGenerateColumns = false;
            List<SpendType> list = sb.GetSpendTypeAll();
            gv_SpendType.DataSource = list;
            
        }
        #endregion

#region 行单击事件
        /// <summary>
        /// 行单击事件
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void gv_SpendType_CellClick(object sender, DataGridViewCellEventArgs e)
        {
            if (e.ColumnIndex != -1)
            {
               SpendType st = (SpendType )gv_SpendType.Rows[e.RowIndex].Tag;//这里报错空引用，Tag为Null何故，高手指教
            }
        }
        #endregion