linq 指定的转换无效

string newshopname = "淘宝" + itemname.Text;
                        string str_f = "account_name='" + newshopname + "'";
                        DataTable dt2 = dt.Select(str_f).CopyToDataTable();
                        dt2.Columns.Remove("account_name");
                        DevExpress.XtraCharts.Web.WebChartControl WCC = new DevExpress.XtraCharts.Web.WebChartControl();
                        var query = from q in dt2.AsEnumerable()
                                    group q by q.Field<int>("IsReturns") into r
                                    select new
                                    {
                                        _isreturn = r.Key,
                                        _count = r.Count()
                                    };


改成

string newshopname = "淘宝" + itemname.Text;
                        string str_f = "account_name='" + newshopname + "'";
                        DataTable dt2 = dt.Select(str_f).CopyToDataTable();
                        dt2.Columns.Remove("account_name");
                        DevExpress.XtraCharts.Web.WebChartControl WCC = new DevExpress.XtraCharts.Web.WebChartControl();
                        var query = from q in dt2.AsEnumerable()
                                    group q by q["IsReturns"] into r
                                    select new
                                    {
                                        _isreturn = r.Key,
                                        _count = r.Count()
                                    };