diff --git a/unifi_protect_backup/missing_event_checker.py b/unifi_protect_backup/missing_event_checker.py
index 24b5f63..6c245f4 100644
--- a/unifi_protect_backup/missing_event_checker.py
+++ b/unifi_protect_backup/missing_event_checker.py
@@ -77,9 +77,11 @@ async def _get_missing_events(self) -> AsyncIterator[Event]:
             if not events_chunk:
                 break  # There were no events to backup
 
-            assert events_chunk[-1].end is not None
-            start_time = events_chunk[-1].end
-            unifi_events = {event.id: event for event in events_chunk}
+            # Filter out on-going events
+            unifi_events = {event.id: event for event in events_chunk if event.end is not None}
+
+            # Next chunks start time should be the end of the oldest complete event in the current chunk
+            start_time = max([event.end for event in unifi_events.values()])
 
             # Get list of events that have been backed up from the database