ExpressJS: Error: Can't set headers after they are sent.

I am trying to add a validation method to my ExpressJS app, however it does not quite seem to work out... I can currently change the username, however if it has an empty String, the app crashes, although it is supposing to send an error. Any suggestions?

var validatePresenceOf = function(val, func) {
    if (typeof val === 'undefined' || val.length < 1) {
        res.json({
            errors: {
                userName: "Your name can't be blank"
            }
        });
    } else {
        return func;
    }
};

validatePresenceOf(req.body.userName, User.findByIdAndUpdate( currentUser._id, {
    local: {
        name                : req.body.userName
    }
}, function (err, user) {

    if (err) throw err;

    res.json({
        user : user
    });
}));