Remove uncategorized post from RSS feed

wp-exclude-uncategorized-posts-RSS.php

add_filter('pre_get_posts','exclude_category');
//exclude uncategorized category from rss
function exclude_category($query) {
  if ( $query->is_feed ) {
    $category_id = -1 * get_cat_ID('Uncategorized');
    $query->set('cat', $category_id);
  }
  return $query;
}