diff --git a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHttpTransport.php b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHttpTransport.php
index 2c90472fc6dc6..17f7a7fcf3364 100644
--- a/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHttpTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/Mailchimp/Transport/MandrillHttpTransport.php
@@ -60,7 +60,7 @@ protected function doSendHttp(SentMessage $message): ResponseInterface
             throw new HttpTransportException(sprintf('Unable to send an email (code %s).', $result['code']), $response);
         }
 
-        $message->setMessageId($result['_id']);
+        $message->setMessageId($result[0]['_id']);
 
         return $response;
     }