链接pymongo时，出现TypeError: documents must be a non-empty list怎么办，我的代码是：
import requestsfrom lxml import etreeimport pymongoimport timefrom pymongo import MongoClientconnection = MongoClient()#创建数据库连接对象db = connection['贴吧']#选择一个数据库handler = db['平行宇宙']#选择一个集合url = 'https://tieba.baidu.com/f?ie=utf-8&kw=%E5%B9%B3%E8%A1%8C%E5%AE%87%E5%AE%99&fr=search%27response = requests.get(url=url)response.encoding='utf-8'response = response.textetree=etree.HTML(response)list_tile = etree.xpath('//div[@class=" card_title_fname"]')a = set(list_tile)insert_list = []for k in range(0,len(a)): username = etree.xpath('//*[@class="frs-author-name j_user_card "]/a/text()') discription = etree.xpath('//div[@class="threadlist_abs threadlist_abs_onlyline "]/text()') lasttime = etree.xpath('//span[@class="threadlist_reply_date pull_right j_reply_data"]/text()') print("内容:",discription) print("用户名:",username) print("日期:",lasttime) insert_list.append({'用户名':username,'内容':discription,'截止时间':lasttime}) print(insert_list)handler.insert_many(insert_list)