Join 子句中其中一个表达式的类型不正确。对“GroupJoin”的调用中的类型推理失败。解决办法

        var query = from d in db.d_necro
                    join g in db.Graves
                    on new { d.graveno, d.garea } equals new { g.gno, g.FgrId } into temp1
                    from g in temp1.DefaultIfEmpty()
                    join k in db.GraveKind
                    on d.gakind equals k.gkId into temp2
                    from k in temp2.DefaultIfEmpty()
                    select new
                    {
                        d,
                        g,
                        k
                    };