diff --git a/tests/Support/StringableObject.php b/tests/Support/StringableObject.php
index b6476cb3c..e2065e593 100644
--- a/tests/Support/StringableObject.php
+++ b/tests/Support/StringableObject.php
@@ -8,7 +8,7 @@
 
 final class StringableObject implements Stringable
 {
-    public function __construct(private string $string)
+    public function __construct(private readonly string $string)
     {
     }