Backbone save() respond with success

I have a properly configured Backbone Model, but when I run the save method like so:

backbone_model.save(null, {

    success: function () { console.log('ok'); },

    error : function () { console.log('not ok'); } 

});

I have a successful POST request sent (and Backbone request is triggered) to my Node instance which responds with nothing except 200 OK like so:

app.post('/backbone', function (req, res) {
    res.send(); // Automatically sends 200 OK
}

But Backbone does not trigger the sync event, and the error callback is executed. I've also tried returning 201 Created, with no success.