diff --git a/src/Plugin/Mail/SendGridMail.php b/src/Plugin/Mail/SendGridMail.php
index e496559e62a065694081ec5b06017d25e5d67b63..5a120721554a4144f5548ad033e8f623487d3af4 100644
--- a/src/Plugin/Mail/SendGridMail.php
+++ b/src/Plugin/Mail/SendGridMail.php
@@ -269,7 +269,7 @@ class SendGridMail implements MailInterface, ContainerFactoryPluginInterface {
     }
 
     $sendgrid_message->addCategories($categories);
-    $personalization0->setSubject($message['subject']);
+    $personalization0->setSubject((string) $message['subject']);
     $from = new From($data['from'], $data['fromname']);
     unset($data);
     # Set the from address and add a name if it exists.