diff --git a/tests/inc/MailerTestCase.php b/tests/inc/MailerTestCase.php
index 4b5cdc5..b260db9 100644
--- a/tests/inc/MailerTestCase.php
+++ b/tests/inc/MailerTestCase.php
@@ -21,7 +21,7 @@ public function testMailer(): void
 		Assert::count(2, $mailer->getMessages(2));
 		Assert::count(3, $mailer->getMessages(3));
 
-		$deletedMessageId = array_keys($mailer->getMessages(2))[0];
+		$deletedMessageId = (string) array_keys($mailer->getMessages(2))[0];
 		$mailer->deleteOne($deletedMessageId);
 		Assert::same(2, $mailer->getMessageCount());