public function collect($post_id, string $key = ''): array
    {
        $post_id = (int) ($post_id ?? get_the_ID());

        if (!$post_id) { // Empty key is OK here.
            return []; // Not possible; empty array.
        }
        $value        = get_post_meta($post_id, $this->key($key), false);
        return $value = !is_array($value) ? [] : $value;
    }