Meta Box for Yoast not working

Hello,

I just installed Meta Box (4.8.7), Meta Box for Yoast (1.1.1) and Yoast SEO (3.4.1) and the plugin doesn't seem to work.

Here is my meta box definition :

$meta_boxes[] = array(
      'title'      => __( 'Informations', 'bn' ),
      'post_types' => 'director',
      'context'    => 'normal',
      'priority'   => 'high',
      'fields' => array(
        array(
          'name'  => __( 'Firstname', 'bn' ),
          'id'    => $prefix . 'firstname',
          'type'  => 'text',
          'add_to_wpseo_analysis' => true
        ),
        array(
          'name'  => __( 'About', 'bn' ),
          'id'    => $prefix . 'about',
          'type'  => 'wysiwyg',
          'add_to_wpseo_analysis' => true
        ),
      )
    );

I have one word in the field 'firstname' and a big text in the field 'about' and Yoast's Metabox still finds 0 words in the text.

Did I do something wrong ? Is it going to be fixed anytime soon ?

Thanks !