diff --git a/src/Data/Membership.php b/src/Data/Membership.php
index 3e632bc..31e4c3d 100644
--- a/src/Data/Membership.php
+++ b/src/Data/Membership.php
@@ -28,7 +28,7 @@ class Membership extends DataTransferObject
     public int $activityId;
 
     #[MapFrom('untergliederungId')]
-    public int $subactivityId;
+    public ?int $subactivityId;
 
     #[MapFrom('gruppierung')]
     public ?string $group;