diff --git a/src/RequestHeaders.php b/src/RequestHeaders.php
index fd08cb9..d700ac6 100644
--- a/src/RequestHeaders.php
+++ b/src/RequestHeaders.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Yiisoft\RequestProvider;
 
 final class RequestHeaders {