Phalcon -> Fatal error: Access to undeclared static property: Phalcon\Di::$_default …public/index.php on line 20

I've installed Phalcon on my server Centos 7 with PHP-FPM 5.6

I'm following the basic instructions at https://docs.phalcon.io/en/latest/reference/tutorial.html and get stuck at the beginning:

Sometimes it displays the correct "Hello!" message, but I've refreshed the page several times and most of the time I get the following error:

Fatal error: Access to undeclared static property: Phalcon\Di::$_default in /var/www/vhosts/playerm8.com/httpdocs/public/index.php on line 20

My public/index.php looks like this:

<?php

use Phalcon\Loader;
use Phalcon\Mvc\View;
use Phalcon\Mvc\Application;
use Phalcon\Di\FactoryDefault;
use Phalcon\Mvc\Url as UrlProvider;
use Phalcon\Db\Adapter\Pdo\Mysql as DbAdapter;

try {

    // Register an autoloader
    $loader = new Loader();
    $loader->registerDirs(array(
        '../app/controllers/',
        '../app/models/'
    ))->register();

    // Create a DI
    $di = new FactoryDefault()
;
    // Setup the view component
    $di->set('view', function () {
        $view = new View();
        $view->setViewsDir('../app/views/');
        return $view;
    });

    // Setup a base URI so that all generated URIs include the "tutorial" folder
    $di->set('url', function () {
        $url = new UrlProvider();
        $url->setBaseUri('/');
        return $url;
    });

    // Handle the request
    $application = new Application($di);

    echo $application->handle()->getContent();

} catch (\Exception $e) {
     echo "Exception: ", $e->getMessage();
}

And my apps/controllers/IndexController.php looks like this:

<?php

use Phalcon\Mvc\Controller;

class IndexController extends Controller
{

    public function indexAction()
    {
        echo "<h1>Hello!</h1>";
    }
}

Any help would be appreciated