diff --git a/tools/gen_upcoming.py b/tools/gen_upcoming.py
index 33b093d..31763f7 100755
--- a/tools/gen_upcoming.py
+++ b/tools/gen_upcoming.py
@@ -44,7 +44,10 @@ def parse_recurring_event(event, start, end):
     logging.info(f"Processing recurring event {event['SUMMARY'].to_ical().decode('utf-8')}")
     dtstart = parse(event["DTSTART"].to_ical())
     rs = rruleset()
-    rs.rrule(rrulestr(event["RRULE"].to_ical().decode("utf-8"), dtstart=dtstart))
+    # dtstart is normalized to naive local time above; pass ignoretz so a
+    # tz-aware UNTIL (RFC 5545 requires UTC for it) is treated as naive too,
+    # otherwise dateutil rejects the naive/aware mismatch.
+    rs.rrule(rrulestr(event["RRULE"].to_ical().decode("utf-8"), dtstart=dtstart, ignoretz=True))
     if "EXDATE" in event.keys():
         for exdate in event["EXDATE"]:
             rs.exdate(parse(exdate.to_ical()))