diff --git a/models/classes/user/LtiUser.php b/models/classes/user/LtiUser.php
index 154d3ec8..87418fb0 100755
--- a/models/classes/user/LtiUser.php
+++ b/models/classes/user/LtiUser.php
@@ -216,7 +216,7 @@ public function refresh()
         // nothing to do
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             self::USER_IDENTIFIER => $this->userUri,