Drupal 8 + Add classes to the menu item

In a preprocess menu function(), I want to add custom classes on my li menu. For this I use this:

$  item['attributes']->addClass('section'); // or  $  item['attributes']->setAttribute('class', array('section')); // or  $  item['attributes']->setAttribute('class', 'section'); 

But, I get in the html code this result:

<li class="Array class"> 

How to get the desired classes on my li?

Thanks 😉