diff --git a/src/TransientCleaner.php b/src/TransientCleaner.php
index 04f0d91..1019c86 100644
--- a/src/TransientCleaner.php
+++ b/src/TransientCleaner.php
@@ -202,7 +202,7 @@ public function set_stored_transients($value) : TransientCleaner
      *
      * @return array|bool           New value
      */
-    public function merge_stored_transients_option_values($value, $old_value, $option)
+    public function merge_stored_transients_option_values($value, $old_value, ?string $option = '')
     {
         // Return `$value` if no previous value.
         if (false === $old_value) {