diff --git a/src/wp-cron.php b/src/wp-cron.php
index 15c6676b77..f73d354444 100644
--- a/src/wp-cron.php
+++ b/src/wp-cron.php
@@ -34,6 +34,8 @@ if ( !defined('ABSPATH') ) {
  * @ignore
  * @since 3.3.0
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @return string|false Value of the `doing_cron` transient, 0|false otherwise.
  */
 function _get_cron_lock() {