Display all Categories

I use this to display all categories:

<?php $args = array(
    'child_of'            => 0,
    'current_category'    => 0,
    'depth'               => 0,
    'echo'                => 1,
    'exclude'             => '',
    'exclude_tree'        => '',
    'feed'                => '',
    'feed_image'          => '',
    'feed_type'           => '',
    'hide_empty'          => 1,
    'hide_title_if_empty' => false,
    'hierarchical'        => true,
    'order'               => 'ASC',
    'orderby'             => 'name',
    'separator'           => ' ',
    'show_count'          => 0,
    'show_option_all'     => '',
    'style'               => 'h3',
    'taxonomy'            => 'category',
    'title_li'            => __( 'Categories' ),
    'use_desc_for_title'  => 1,
); 

var_dump( wp_list_categories($args) );  ?>

But it also gives me back the word "NULL".

How can I remove the "NULL"?