C# orderby无法使用LINQ在子查询中获取前1个结果

我正在尝试使用子查询从子表中获取前1个结果。当我尝试使用

orderby或OrderByDescending

使用子查询时，它会抛出

NullReferenceException

。不使用

orderby或OrderByDescending

我的查询将成功执行。 但我想得到子查询的最后一条记录。我的应用程序与Mysql数据库连接

我的代码如下

var result = (from ud in db.user_devices
                          join a in db.access_info on ud.u_id equals a.u_id
                          join d in db.device_num on ud.dev_id equals d.DevId
                          //let pi = db.packet_info.Where(x =>  x.DevId == ud.dev_id && x.DevId != null).OrderByDescending(x =>x.systime).FirstOrDefault()
                          //where  pi != null
                          select new
                          {
                              fuel = db.packet_info.Where(x => x.DevId == ud.dev_id).OrderByDescending(x => x.systime).Select(x => x.fuel).FirstOrDefault(),
                              //removed other fields
                          }).ToList();

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使用另一种方法，创建了一个与linq模式相同的过程

程序代码如下所示：

CREATE DEFINER=`abc`@`%` PROCEDURE `spGetAllVechiles`()
BEGIN
    SELECT 

       (SELECT g.fuel  FROM abc.Packet_Info AS g WHERE g.devid = ud.dev_id order by systime desc LIMIT 0,1 ) AS fuel,
       (SELECT  g.fencealarm FROM abc.Packet_Info AS g WHERE g.devid = ud.dev_id order by systime desc LIMIT 0,1 ) AS fencealarm,
       (SELECT  g.speed  FROM abc.Packet_Info AS g WHERE g.devid = ud.dev_id order by systime desc LIMIT 0,1 ) AS speed,
       (SELECT  g.Latitude FROM abc.Packet_Info AS g WHERE g.devid = ud.dev_id order by systime desc LIMIT 0,1 ) AS Latitude,
       (SELECT  g.Longitude FROM abc.Packet_Info AS g WHERE g.devid = ud.dev_id order by systime desc LIMIT 0,1 ) AS Longitude
       //other params...

    FROM abc.access_info AS ac 
    INNER JOIN abc.user_devices AS ud ON ud.u_id = ac.u_id
    INNER JOIN abc.device_num AS dn ON dn.DevId = ud.dev_id;

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请提供db.packet_info的定义或至少systime属性的类型？其数据类型在mysql中为timestamp，在C中为DateTime？systime中是否有可能为空？没有空值。。我查过了。。