diff --git a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
index d614fdbcdcd..3d61d2e618f 100644
--- a/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ModuleHandlerInterface.php
@@ -237,7 +237,9 @@ interface ModuleHandlerInterface {
    *
    * @return array
    *   An array of return values of the hook implementations. If modules return
-   *   arrays from their implementations, those are merged into one array.
+   *   arrays from their implementations, those are merged into one array
+   *   recursively. Note: integer keys in arrays will be lost, as the merge is
+   *   done using array_merge_recursive().
    */
   public function invokeAll($hook, array $args = array());