diff --git a/src/FlashMessageMiddleware.php b/src/FlashMessageMiddleware.php
index 6a0702a..c7cdbde 100644
--- a/src/FlashMessageMiddleware.php
+++ b/src/FlashMessageMiddleware.php
@@ -22,8 +22,8 @@ class FlashMessageMiddleware implements MiddlewareInterface
 
     public function __construct(
         string $flashMessagesClass = FlashMessages::class,
-        private string $sessionKey = FlashMessagesInterface::FLASH_NEXT,
-        private string $attributeKey = self::FLASH_ATTRIBUTE
+        private readonly string $sessionKey = FlashMessagesInterface::FLASH_NEXT,
+        private readonly string $attributeKey = self::FLASH_ATTRIBUTE
     ) {
         $factory = [$flashMessagesClass, 'createFromSession'];
         if (! is_callable($factory)) {
diff --git a/src/FlashMessages.php b/src/FlashMessages.php
index 4ab3c62..801ab3f 100644
--- a/src/FlashMessages.php
+++ b/src/FlashMessages.php
@@ -32,7 +32,7 @@ class FlashMessages implements FlashMessagesInterface
     /** @var array<string,mixed> */
     private array $currentMessages = [];
 
-    private function __construct(private SessionInterface $session, private string $sessionKey)
+    private function __construct(private readonly SessionInterface $session, private readonly string $sessionKey)
     {
         $this->prepareMessages($session, $sessionKey);
     }