diff --git a/PickPoint.php b/PickPoint.php
index 52fd849..ff85153 100644
--- a/PickPoint.php
+++ b/PickPoint.php
@@ -139,7 +139,7 @@ public function getName(): string
         return 'pickpoint';
     }
 
-    private function executeQuery(string $url, string $locale = null): string
+    private function executeQuery(string $url, ?string $locale = null): string
     {
         if (null !== $locale) {
             $url = sprintf('%s&accept-language=%s', $url, $locale);