diff --git a/src/Matchers/ReverseDictionaryMatch.php b/src/Matchers/ReverseDictionaryMatch.php
index 4f38ecd..a935bff 100644
--- a/src/Matchers/ReverseDictionaryMatch.php
+++ b/src/Matchers/ReverseDictionaryMatch.php
@@ -55,7 +55,7 @@ public function getFeedback(bool $isSoleMatch): array
         return $feedback;
     }
 
-    public static function mbStrRev(string $string, string $encoding = null): string
+    public static function mbStrRev(string $string, ?string $encoding = null): string
     {
         if ($encoding === null) {
             $encoding = mb_detect_encoding($string) ?: 'UTF-8';