diff --git a/modules/aggregator.module b/modules/aggregator.module
index aebd5fca376..afdf0160bd1 100644
--- a/modules/aggregator.module
+++ b/modules/aggregator.module
@@ -245,7 +245,7 @@ function aggregator_cron() {
  *
  * Generates blocks for the latest news items in each category and feed.
  */
-function aggregator_block($op, $delta = 0, $edit = array()) {
+function aggregator_block($op = 'list', $delta = 0, $edit = array()) {
   if (user_access('access news feeds')) {
     if ($op == 'list') {
       $result = db_query('SELECT cid, title FROM {aggregator_category} ORDER BY title');