diff --git a/lib/mail/Mail.php b/lib/mail/Mail.php
index c8511852..3c1f446b 100644
--- a/lib/mail/Mail.php
+++ b/lib/mail/Mail.php
@@ -101,7 +101,7 @@ public function __construct(
         $subject = null,
         $plainTextContent = null,
         $htmlContent = null,
-        array $globalSubstitutions = null
+        ?array $globalSubstitutions = null
     ) {
         if (!isset($from)
             && !isset($to)