diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2dd5338..c2bbd8d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+1.3.1
+=====
+
+* (improvement) Make `SimpleNormalizer` non-readonly, to allow it to be mocked.
+
+
 1.3.0
 =====
 
diff --git a/src/Normalizer/SimpleNormalizer.php b/src/Normalizer/SimpleNormalizer.php
index 38ac9b6..e62c3ba 100644
--- a/src/Normalizer/SimpleNormalizer.php
+++ b/src/Normalizer/SimpleNormalizer.php
@@ -9,15 +9,19 @@
 use Torr\SimpleNormalizer\Exception\UnsupportedTypeException;
 
 /**
+ * The normalizer to use in your app.
+ *
+ * Can't be readonly, as it needs to be mock-able.
+ *
  * @final
  */
-readonly class SimpleNormalizer
+class SimpleNormalizer
 {
 	/**
 	 * @param ServiceLocator<SimpleObjectNormalizerInterface> $objectNormalizers
 	 */
 	public function __construct (
-		private ServiceLocator $objectNormalizers,
+		private readonly ServiceLocator $objectNormalizers,
 	) {}
 
 	/**