Node.js express application. Using mongoose to connect to mongodb. Mongodb is replica set.

Sometimes, the following error happens.

0|worker | TypeError: Cannot read property 'wireProtocolHandler' of null
0|worker | at nextFunction (/var/www/app/node_modules/mongoose/node_modules/mongodb-core/lib/cursor.js:577:31)
0|worker | at Cursor.next [as _next] (/var/www/app/node_modules/mongoose/node_modules/mongodb-core/lib/cursor.js:701:3)
0|worker | at nextObject (/var/www/app/node_modules/mongoose/node_modules/mongodb/lib/cursor.js:680:8)
0|worker | at Cursor.next (/var/www/app/node_modules/mongoose/node_modules/mongodb/lib/cursor.js:270:12)
0|worker | at findOne (/var/www/app/node_modules/mongoose/node_modules/mongodb/lib/collection.js:1414:10)
0|worker | at Collection.findOne (/var/www/app/node_modules/mongoose/node_modules/mongodb/lib/collection.js:1400:44)
0|worker | at NativeCollection.(anonymous function) [as findOne] (/var/www/app/node_modules/mongoose/lib/drivers/node-mongodb-native/collection.js:130:28)
0|worker | at NodeCollection.findOne (/var/www/app/node_modules/mongoose/node_modules/mquery/lib/collection/node.js:42:19)
0|worker | at model.Query.Query.findOne (/var/www/app/node_modules/mongoose/node_modules/mquery/lib/mquery.js:1816:20)
0|worker | at model.Query.Query._findOne (/var/www/app/node_modules/mongoose/lib/query.js:1519:22)
0|worker | at /var/www/app/node_modules/mongoose/node_modules/kareem/index.js:250:8
0|worker | at /var/www/app/node_modules/mongoose/node_modules/kareem/index.js:23:7
0|worker | at _combinedTickCallback (internal/process/next_tick.js:73:7)
0|worker | at process._tickDomainCallback (internal/process/next_tick.js:128:9)