diff --git a/src/WhatsAppCloudApi.php b/src/WhatsAppCloudApi.php
index 7383319..d8842e0 100644
--- a/src/WhatsAppCloudApi.php
+++ b/src/WhatsAppCloudApi.php
@@ -95,7 +95,7 @@ public function sendTextMessage(string $to, string $text, bool $preview_url = fa
      *
      * @throws Response\ResponseException
      */
-    public function sendDocument(string $to, MediaID $document_id, string $name, ?string $caption): Response
+    public function sendDocument(string $to, MediaID $document_id, string $name, ?string $caption = ''): Response
     {
         $message = new Message\DocumentMessage($to, $document_id, $name, $caption, $this->reply_to);
         $request = new Request\MessageRequest\RequestDocumentMessage(