列表插件给单据体的基础资料赋值时报错列表被标记为只读

A单据列表插件，将A单据头的多选基础资料赋值到B单据的单据体中时，提示错误列表被标记为只读
FormMetadata meta = MetaDataServiceHelper.Load(this.Context, "QX_LOADTASK") as FormMetadata;//被赋值单据
BusinessInfo _info = meta.BusinessInfo;
DynamicObject _currentChangeObj = new DynamicObject(_info.GetDynamicObjectType());
DynamicObjectCollection entryLoadingGoods = _currentChangeObj["FLoadingGoods"] as DynamicObjectCollection;//被赋值单据体数据包
DynamicObject entryObj = new DynamicObject(_info.GetEntryEntity("FLoadingGoods").DynamicObjectType);
entryObj["FLOADINGGOODSNAME"] = item["FGOODSNAME"];//报错行
entryObj["FLOADINGVEHICLE_Id"] = (item["FVEHICLE"] as DynamicObject)["Id"];
entryObj["FLOADINGVHNUM"] = item["FCONTAINERNUM"];
entryLoadingGoods.Add(entryObj);