请问老师，登录后一定的几率出现这个错误是为什么？是connect-mongo的BUG吗？还是我用的问题？
Error: Error setting TTL index on collection : sessions at D:\WorkSource\Imooc\node_modules\connect-mongo\lib\connect-mongo.js:161:23 at D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:1404:28 at D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:1542:30 at D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\cursor.js:159:22 at commandHandler (D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\cursor.js:678:48) at D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:1806:9 at __executeQueryCommand (D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:1636:44) at Db._executeQueryCommand (D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\db.js:1805:7) at Cursor.nextObject (D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\cursor.js:729:13) at Cursor.toArray (D:\WorkSource\Imooc\node_modules\connect-mongo\node_modules\mongodb\lib\mongodb\cursor.js:158:10)
