diff --git a/aleksis/apps/chronos/models.py b/aleksis/apps/chronos/models.py
index 7f4b35959b438bc803ff67dfcf56271f7b2683fc..8cb324f591a48a7a18b211f9d4e091375fd84062 100644
--- a/aleksis/apps/chronos/models.py
+++ b/aleksis/apps/chronos/models.py
@@ -1640,7 +1640,7 @@ class LessonEvent(CalendarEvent):
             )
 
             if affected_teachers.exists():
-                existing_substitutions = reference_obj.amended_by.filter(
+                existing_substitutions = reference_obj.amended_by.instance_of(cls).filter(
                     datetime_start=event["DTSTART"].dt,
                     datetime_end=event["DTEND"].dt,
                 )