Node.js MongoDB Sockets closed error

I'm currently making a login system for my NodeJS application. However, I get a strange error from MongoDB whenever I try retrieving a collection.

Error Message

[MongoError: server localhost:27017 sockets closed]
name: 'MongoError',
message: 'server localhost:27017 sockets closed'

Heres my code to connect to my db

    var username = req.body.user.username;
    var password = req.body.user.password;

    MongoClient.connect("mongodb://localhost:27017/myDb", function(err, db){
        assert.equal(null, err);

        var collection = db.collection("accounts");
        collection.findOne({"username": username}, function(err, item){
            console.log(item);
            console.log(err);
        });

        db.close();
    });

Is anyone able to see where Ive gone wrong?