diff --git a/src/Format/Indent.php b/src/Format/Indent.php
index 4adae936..22550f47 100644
--- a/src/Format/Indent.php
+++ b/src/Format/Indent.php
@@ -16,6 +16,9 @@
 use Ergebnis\Json\Normalizer\Exception;
 use Ergebnis\Json\Normalizer\Json;
 
+/**
+ * @psalm-immutable
+ */
 final class Indent
 {
     public const CHARACTERS = [