diff --git a/Lock.php b/Lock.php
index f529c66..1db5821 100644
--- a/Lock.php
+++ b/Lock.php
@@ -54,7 +54,7 @@ public function __sleep(): array
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }
 
-    public function __wakeup()
+    public function __wakeup(): void
     {
         throw new \BadMethodCallException('Cannot unserialize '.__CLASS__);
     }