无法将类型“System.Nullable`1”强制转换为类型“System.Object”。LINQ to Entities 仅支持强制转换 EDM 基元或枚举类型。

 public ActionResult GetIpSegments()
        {
            //List<Ipsegment> ipsegments = (from s in deviceDB.Ipsegment select s).ToList();
            var ipsegments = from d in deviceDB.DeviceCategory
                             join s in deviceDB.Ipsegment
                             on d.devicecategoryid equals s.devicecategoryid
                             select new
                                        {
                                            devicecategoryid1 = s.devicecategoryid,
                                            devicecategoryname1 = d.devicecategoryname,
                                            ipsegment = "202.115.242." + s.ip_head + "-202.115.242." + s.ip_end
                                        };

            return Json(ipsegments, JsonRequestBehavior.AllowGet);
        }

    public ActionResult GetIpSegments()
        {
            var ipsegments = from s in deviceDB.Ipsegment
                             select new
                             {
                                 s.devicecategoryid,
                                 s.DeviceCategory.devicecategoryname,
                                 s.ip_head,
                                 s.ip_end
                             };

            return Json(ipsegments, JsonRequestBehavior.AllowGet);
        }