diff --git a/livesync/indico_livesync/handler.py b/livesync/indico_livesync/handler.py
index a35f094..f837c10 100644
--- a/livesync/indico_livesync/handler.py
+++ b/livesync/indico_livesync/handler.py
@@ -153,11 +153,14 @@ def _register_deletion(obj, parent):
 
 
 def _register_change(obj, action):
-    if isinstance(obj, Conference) and (ConferenceHolder().getById(obj.id, True) is None or obj.getOwner() is None):
-        # When deleting an event we get data change signals afterwards. We can simple ignore them.
-        # When moving an event it's even worse, we get a data change notification in the middle of the move while the
-        # event has no category...
-        return
+    if not isinstance(obj, Category):
+        event = obj.getConference()
+        if event is None or ConferenceHolder().getById(event.id, True) is None or event.getOwner() is None:
+            # When deleting an event we get data change signals afterwards. We can simple ignore them.
+            # When moving an event it's even worse, we get a data change notification in the middle of the move while
+            # the event has no category...
+            # Also, ACL changes during user merges might involve deleted objects which we also don't care about
+            return
     _init_livesync_g()
     g.livesync_changes[obj_ref(obj)].add(action)