How do you get $bot->userStorage()->delete() to work in botman?

Question

Considering the code below:

$botman->hears('show-delete', function($bot) {
    $stuff = $bot->userStorage()->all();
    $bot->reply(serialize($stuff)); // first dump
    $bot->userStorage()->delete();
    $stuff = $bot->userStorage()->all();
    $bot->reply(serialize($stuff)); // shows exactly same stuff as first dump
}); 

If the manual and this reply are correct, the second reply statement should be an empty object but its exactly the same. In other words, the delete method has no effect.

What could be wrong?