diff --git a/includes/common.inc b/includes/common.inc
index bb27d391e8b072fffa27c333873ba7f7ae92031e..34fa9b9620c6236d2b3d4709de5b98c413c37ca9 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -5323,8 +5323,8 @@ function drupal_page_set_cache() {
  *
  * Do not call this function from a test. Use $this->cronRun() instead.
  *
- * @return
- *   TRUE if cron ran successfully.
+ * @return bool
+ *   TRUE if cron ran successfully and FALSE if cron is already running.
  */
 function drupal_cron_run() {
   // Allow execution to continue even if the request gets canceled.