diff --git a/src/Options.php b/src/Options.php
index 02b258c..02b2cc8 100644
--- a/src/Options.php
+++ b/src/Options.php
@@ -1027,7 +1027,7 @@ public function writeThumbnail(bool $writeThumbnail): self
 
     /**
      * Convert thumbnail to another format.
-     * supported: jpg, png, webp
+     * @param 'jpg'|'png'|'webp'|null $format
      */
     public function convertThumbnail(?string $format): self
     {