Remove lowest value from array mongodb (Nodejs)

I'm trying to remove the lowest value from each student with this code i have made so far:

// Find smallest value
Array.prototype.min = function() {
  return Math.min.apply(null, this);
};


var MongoClient = require('mongodb').MongoClient;


MongoClient.connect('mongodb://localhost:27017/school', function(err, db) {
    if(err) throw err;

    var collection = db.collection('students');
    var cursor = collection.find();

    cursor.each(function(err, docs){
        if(err) throw err;
        if(docs == null) throw err;
        var student = docs;
        var studentHWScores = [];
        console.log("   ");
        for (var i = 0; i < docs.scores.length; i++) {
            var score = docs.scores[i];
            if (score.type == 'homework') {
                studentHWScores.push(score.score);
            };
        };
        var studentId = student._id.toString();
        var lowestScore = studentHWScores.min().toString();


        // If equal 2 or more in grades remove lowest
        //console.log("studentHWScores.length >= 2: ", studentHWScores.length >= 2);
        if(studentHWScores.length >= 2) {
            collection.update({
                '_id': studentId
            }, {
                '$pull': {
                    'scores': {
                        'score': lowestScore
                    }
                }
            }, function(err, updated) {
                if(err) throw err;
                console.log("updated: ", updated);
            });
        }
        console.log("   ");
    });
});

I get the following error when im trying to run it: (i also get a lot of white space and then (updated: 0) trough my console.log

/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/server.js:500
        throw err;
              ^
TypeError: Cannot read property 'err' of null
    at Object.toError (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/utils.js:113:18)
    at Server.Base._callHandler (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/base.js:455:65)
    at /Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/server.js:487:18
    at MongoReply.parseBody (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/responses/mongo_reply.js:68:5)
    at null.<anonymous> (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/server.js:445:20)
    at emit (events.js:95:17)
    at null.<anonymous> (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:207:13)
    at emit (events.js:98:17)
    at Socket.<anonymous> (/Users/sp/Documents/dev/felicity/node_modules/mongodb/lib/mongodb/connection/connection.js:440:22)
    at Socket.emit (events.js:95:17)