Filling post title with custom fields

I have a custom post type and a couple of custom fields. I tried to fill the post title with the format “song_title – song_artist” with following code but failed:

function frontend_update_post_title($post_id) {
	if ( get_post_type() == 'chart' ) {
		$postdata = array(
			'post_title'  => get_field('song_title', $post_id).' - '.get_field('song_artist', $post_id),
		);
	}
	wp_update_post( $postdata );
}
add_action( 'wpuf_add_post_after_insert', 'frontend_update_post_title' );
add_action( 'wpuf_edit_post_after_update', 'frontend_update_post_title' );

What am I missing?