diff --git a/core/lib/Drupal/Core/Entity/EntityChangedInterface.php b/core/lib/Drupal/Core/Entity/EntityChangedInterface.php
index c0f89702e9813f573f60e8cfaffa8a080e41e362..424e6f6c9465d801bf664d73bb557301abcacd0e 100644
--- a/core/lib/Drupal/Core/Entity/EntityChangedInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityChangedInterface.php
@@ -21,8 +21,9 @@ interface EntityChangedInterface extends EntityInterface {
   /**
    * Gets the timestamp of the last entity change for the current translation.
    *
-   * @return int
-   *   The timestamp of the last entity save operation.
+   * @return int|null
+   *   The timestamp of the last entity save operation. Some entities allow a
+   *   NULL value indicating the changed time is unknown.
    */
   public function getChangedTime();