diff --git a/includes/api/ApiEmailUser.php b/includes/api/ApiEmailUser.php
index b91e30254fc..b68a1620473 100644
--- a/includes/api/ApiEmailUser.php
+++ b/includes/api/ApiEmailUser.php
@@ -95,7 +95,10 @@ class ApiEmailUser extends ApiBase {
 				ApiBase::PARAM_TYPE => 'string',
 				ApiBase::PARAM_REQUIRED => true
 			],
-			'subject' => null,
+			'subject' => [
+				ApiBase::PARAM_TYPE => 'string',
+				ApiBase::PARAM_REQUIRED => true
+			],
 			'text' => [
 				ApiBase::PARAM_TYPE => 'text',
 				ApiBase::PARAM_REQUIRED => true