package_quantity是2，接口返回waybill_id_list子母单号列表是空的，请问这是什么原因？

{"ewaybill_order_id":"1777951493326xxxxx","delivery_id":"JD","site_code":null,"ewaybill_acct_id":"66439xxxxx","sender":{"name":"刘政锴","mobile":"176885xxxxx","province":"广东省","city":"肇庆市","county":"四会市","street":null,"address":"肇庆高新技术产业开发区大旺高新区龙湖大道18号钜益工业园东门(6号楼)二楼"},"receiver":{"name":"蔡**","mobile":"1**********","province":"湖南省","city":"永州市","county":"东安县","street":"","address":"****"},"ec_order_list":[{"ec_order_id":37361647518712xxxxx,"goods_list":[{"good_name":"15寸低音爱多IDALL户外广场舞音响带显示屏家用K歌直播一体机蓝牙音箱","good_count":1,"product_id":10000536293759,"sku_id":8706846381,"out_product_id":"","out_sku_id":"","out_goods_info":null}],"ewaybill_order_code":null,"ewaybill_order_appid":null}],"remark":"需要改地址：：湖南省永州市东安县水岭乡唐高雄转蔡老师收。电话18174226798","shop_id":"5009839874175","return_address":{"name":"刘政锴","mobile":"17688587418","province":"广东省","city":"肇庆市","county":"四会市","street":null,"address":"肇庆高新技术产业开发区大旺高新区龙湖大道18号钜益工业园东门(6号楼)二楼"},"template_id":"6642816464","order_type":12,"order_vas_list":null,"ext_info":null,"delivery_info":{"delivery_type":0,"collected_time_begin":0,"collected_time_end":0,"package_quantity":2}}