Android : Timer behaving strangely in Android

I'm setting a Timer to update my Fragment, with the code

private List<Item> items;
...

autoUpdate = new Timer();
    autoUpdate.schedule(new TimerTask() {
        @Override
        public void run() {
            update();
        }
    }, seconds*1000, seconds*1000);

public void update() {
    if (items == null){
        Log.d("test", "items are null");
    }       
}

When the Timer calls the update method, items always appear as null. However if I call the same method in any other way (like by pressing a button), the same items do not appear as null! Any ideas?