左连接出现LINQ错误

我是linq的新手，所以我会感谢你的帮助，让这个左连接查询工作。

我有"列表小时"(0-23小时的列表)，我想与"var hourlyData"(包含聚合数据的处理linq查询)左连接。我在左连接的尝试是"var reportData"。

DataSet ds = GetDataSet(sSql);
var stats = ds.Tables[0].AsEnumerable();
var hourlyData = from stat in stats
                 group stat by stat.Field<int>("Hour") into g
                 select new
                 {
                     Hour = g.Key,
                     Value = g.Key.ToString(),
                     D1 = g.Sum(stat => stat.Field<int>("D1")),
                     D2 = g.Sum(stat => stat.Field<int>("D2")),
                     D3 = g.Sum(stat => stat.Field<decimal>("D3"))
                 };

List<int> hours = new List<int>();
for (int i = 0; i < 24; i++)
{
    hours.Add(i);
}

var reportData = from hour in hours.AsEnumerable()
                 join stat in hourlyData.AsEnumerable()
                     on hour equals stat.Hour 
                 into sts2
                 from stat2 in sts2.DefaultIfEmpty()
                 select new
                 {
                     Hour = hour,
                     Value = hour,
                     D1 = stat2.D1 != null ? stat2.D1 : 0,
                     D2 = stat2.D2 != null ? stat2.D2 : 0,
                     D3 = stat2.D3 != null ? stat2.D3 : 0
                 };

上面的代码会产生如下错误:

Exception Details: System。NullReferenceException:对象引用没有设置为对象的实例。

源错误:

Line 135:                            into sts2
Line 136:                         from stat2 in sts2.DefaultIfEmpty()
Line 137:                         select new
Line 138:                         {
Line 139:                             Hour = hour,
 ...

谢谢!