diff --git a/Behavioral/Observer/UserObserver.php b/Behavioral/Observer/UserObserver.php
index 417ad81..2630648 100644
--- a/Behavioral/Observer/UserObserver.php
+++ b/Behavioral/Observer/UserObserver.php
@@ -15,7 +15,7 @@ class UserObserver implements SplObserver
     /**
      * It is called by the Subject, usually by SplSubject::notify()
      */
-    public function update(SplSubject $subject)
+    public function update(SplSubject $subject): void
     {
         $this->changedUsers[] = clone $subject;
     }