diff --git a/server/vbv_lernwelt/assignment/services.py b/server/vbv_lernwelt/assignment/services.py
index 9e93c865..5f9ac435 100644
--- a/server/vbv_lernwelt/assignment/services.py
+++ b/server/vbv_lernwelt/assignment/services.py
@@ -129,7 +129,8 @@ def update_assignment_completion(
         substasks = assignment.get_input_tasks()
         for key, value in ac.completion_data.items():
             task_data = find_first(substasks, pred=lambda x: x["id"] == key)
-            ac.completion_data[key].update(task_data)
+            if task_data:
+                ac.completion_data[key].update(task_data)
 
     ac.save()
 
@@ -151,7 +152,8 @@ def update_assignment_completion(
         subtasks = assignment.get_input_tasks()
         for key, value in acl.completion_data.items():
             task_data = find_first(subtasks, pred=lambda x: x["id"] == key)
-            acl.completion_data[key].update(task_data)
+            if task_data:
+                acl.completion_data[key].update(task_data)
         acl.save()
 
     return ac