【WordPress】現在の投稿のカテゴリに属する投稿一覧を取得して表示

function list_post( $args ){
	
	global $post;
	$my_category = get_the_category( $post->ID );
	
	$defaults = array(
	'posts_per_page'  => 5,
	);
	$atts = shortcode_atts( $defaults , $args , 'list_posts' );
	
	if( is_single() ){
		
		$post_args = array(
		'posts_per_page' => $atts[ 'posts_per_page' ],
		'category_name' =>  $my_category[0]->slug,
		);
		
		$my_posts = get_posts( $post_args );
		
		echo '<ul>';
		foreach( $my_posts as $post ){
			setup_postdata( $post );
			$output = '<li>' . get_the_title() . '</li>';
			echo $output;
		}
		wp_reset_postdata();
		echo '</ul>';
		
	}
	
}
add_shortcode( 'list_posts' , 'list_post' );