Problem displaying posts for a category in a custom taxonomy

$args = array(
    'posts_per_page' => get_option('posts_per_page'),   
    'post_type' => 'listing',
    'tax_query' => array(
        array(
            'taxonomy' => 'listing_category',
            'field' => 'slug', //can be set to ID
            'terms' => array(81) 
        )
    )
);
$query = new WP_Query( $args );