diff --git a/core/lib/Drupal/Component/Render/MarkupInterface.php b/core/lib/Drupal/Component/Render/MarkupInterface.php
index 41c4f3adb1..0baeff1a9a 100644
--- a/core/lib/Drupal/Component/Render/MarkupInterface.php
+++ b/core/lib/Drupal/Component/Render/MarkupInterface.php
@@ -47,6 +47,6 @@ public function __toString();
    * @return int
    *   The length of the string.
    */
-  public function count();
+  public function count(): int;
 
 }