Node server crash with mongo replica connection

Question

I am connecting with my mongo replica set using the set of code

    var mongojs = require('mongojs');
    var replicationSet = "primary:27017,secondary:27017";
    var replicaSetName = "rs";
    var readPreference = "nearest";
    var readPreferenceTags = "dc:test";
    var generateMongoUri = function (username, pswd, db, authdb) {
        var url = format("mongodb://%s:%s@%s/%s?authSource=%s&replicaSet=%s&readPreference=%s&readPreferenceTags=%s&connectTimeoutMS=%s&socketTimeoutMS=%s", username, pswd, replicationSet, db, authdb, replicaSetName, readPreference, readPreferenceTags, 5000, 5000);
        return url;
    };
    var userurl = generateMongoUri(username,username, pswd, db, authdb);
    var db = mongojs(userurl);

Not often but sometimes i get this error and my node crashes and I don't even understand why this error has popped

E:\NodeServer\node_modules\mongodb\lib\db.js:309
        handleCallback(callback, null, result.result);
                                         ^

TypeError: Cannot read property 'result' of undefined
    at E:\NodeServer\V122\node_modules\mongodb\lib\db.js:309:42
    at ReplSet.command (E:\NodeServer\V122\node_modules\mongodb-core\lib\topologies\replset.js:1094:
42)
    at ReplSet.command (E:\NodeServer\V122\node_modules\mongodb\lib\replset.js:388:18)
    at Store.execute (E:\NodeServer\V122\node_modules\mongodb\lib\topology_base.js:108:30)
    at done (E:\NodeServer\V122\node_modules\mongodb-core\lib\topologies\replset.js:267:40)
    at null.<anonymous> (E:\NodeServer\V122\node_modules\mongodb-core\lib\topologies\replset.js:342:
9)
    at g (events.js:260:16)
    at emitOne (events.js:77:13)
    at emit (events.js:169:7)
    at E:\NodeServer\V122\node_modules\mongodb-core\lib\topologies\server.js:283:14
    at E:\NodeServer\V122\node_modules\mongodb-core\lib\connection\pool.js:436:18
    at nextTickCallbackWith0Args (node.js:420:9)
    at process._tickCallback (node.js:349:13)

Can anyone tell me in what scenario this error gets popped up???