diff --git a/Creational/Prototype/BookPrototype.php b/Creational/Prototype/BookPrototype.php
index 5175b15..accd898 100644
--- a/Creational/Prototype/BookPrototype.php
+++ b/Creational/Prototype/BookPrototype.php
@@ -14,7 +14,7 @@ abstract class BookPrototype
         return $this->title;
     }
 
-    public function setTitle(string $title)
+    public function setTitle(string $title): void
     {
         $this->title = $title;
     }