diff --git a/src/Composer/PHPStan/ConfigReturnTypeExtension.php b/src/Composer/PHPStan/ConfigReturnTypeExtension.php
index 574d7f921..9c769d736 100644
--- a/src/Composer/PHPStan/ConfigReturnTypeExtension.php
+++ b/src/Composer/PHPStan/ConfigReturnTypeExtension.php
@@ -1,4 +1,4 @@
-<?php
+<?php declare(strict_types=1);
 
 namespace Composer\PHPStan;