diff --git a/app/Mail/CheckoutAssetMail.php b/app/Mail/CheckoutAssetMail.php
index d14a2430f4..ba79e59ef9 100644
--- a/app/Mail/CheckoutAssetMail.php
+++ b/app/Mail/CheckoutAssetMail.php
@@ -25,7 +25,7 @@ class CheckoutAssetMail extends BaseMailable
      * Create a new message instance.
      * @throws \Exception
      */
-    public function __construct(Asset $asset, $checkedOutTo, User $checkedOutBy, $acceptance, $note, bool $firstTimeSending = true)
+    public function __construct(Asset $asset, $checkedOutTo, ?User $checkedOutBy, $acceptance, $note, bool $firstTimeSending = true)
     {
         $this->item = $asset;
         $this->admin = $checkedOutBy;