Linq中带连接的多个条件

对于下面的代码，它给出了错误

type reference failed in the call to Join

如何解决这个问题

        var unp = from v in context.student.Include("subjects").Include("marks")
                  join n in context.AllStudents 
                    on v.StudentDetails.student_Id equals n.Id
                  join ss in context.StudentHistory 
                    on v.StBacklogs_Id equals ss.Id
                  join userName in context.Users 
                    on v.CreatedBy_Id equals userName.Id
                  join externalId in context.Books 
                    on new { ss.BookNumber, ss.Id } equals new { externalId.BookNumber, externalId.Id }                  
                  select new { v, n, ss, userName,externalId };

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完整的错误消息将有助于在第4条join语句中显示错误。与多个条件比较，其中一个属性（BookNumber，Id）可为null，而匹配的属性不可为null？将设置peoperties，并且在执行join或PROJECTION时将忽略不可为null的

Include

语句