异常: MongoError: cannot establish topology capabilities as driver is still in process of connecting

var mongodb = require('mongodb');
var db = new mongodb.Db('image_test', new mongodb.Server('127.0.0.1', 27017));

var ImageDB = {
    'save_image': function(img, callback){
        var gs = new mongodb.GridStore(db, name, 'w',{
            'content_type' : img.mimetype
        });
        gs.open(function(err, gs){
            gs.write(img.buffer, function(){
                gs.close(function(err, result){
                    if(err){
                        console.log(err);
                    }else{
                        console.log(result);
                    }
                });
            });
        });
    },
}

请问这何解，今天刚学node