linq DataSet group 多字段 出现：必须至少有一个对象实现 IComparable 的错误。请我应该怎样解决？？

Question

var zq =
                from p in sGERPDataSet.CWTMP
                group p by new { p.Flag, p.YWB_HXT, p.YWB_YWID, p.YWB_DJID } into g
                orderby g.Key
                select
            new
            {
                g.Key,
                NumProducts = g.Count(),
                g
            };

            foreach (var _v in zq)     //执行到这里，出现：必须至少有一个对象实现 IComparable  的错误。请我应该怎样解决？？
            {
                MessageBox.Show(_v.Key.ToString());

                MessageBox.Show(_v.NumProducts.ToString());

                foreach (var _vv in _v.g)
                {
                    MessageBox.Show(_vv.Flag.ToString() + _vv.YWB_HXT.ToString());
                }

            }