使用左连接组的组连接返回null

Question

我有一个有2个连接组的组连接，其中一个组连接可以为null，在我的查询中C是我用左连接加入的组此查询给我和null异常

from A in contexto.A.Where(i => i.EmpresaId == id)                             
                            join B in contexto.B
                                on A.Id equals B.AId
                            join D in contexto.D
                                on B.BId equals D.Id
                           join C in contexto.C
                                on A.Id equals C.AId
                           into c
                           from C in c.DefaultIfEmpty()

                           group new { A, C, D} by A into grupo
                           select new ADTO{
                               Clave = grupo.Key.Clave,
                               Nombre = grupo.Key.Nombre,
                               Lista1 = grupo.Select(t =>                                    
                                 t.D.Nombre                                     
                               ).ToList(),
                               Valores1 = grupo.Select(t => new ValorDTO
                               {
                                   a= t.C.A,
                                   b= t.C.B,
                                   c= t.C.c
                               }
                               ).DefaultIfEmpty(new ValorBMDTO()).ToList()
                           }).ToList();