diff --git a/update.py b/update.py
index a22b169..30b65a7 100755
--- a/update.py
+++ b/update.py
@@ -65,6 +65,12 @@ def update_gwr_advance_ticket_date(config: flask.config.Config) -> None:
     new_html = requests.get(agenda.gwr.url).text
 
     new_dates = agenda.gwr.extract_dates(new_html)
+    if not new_dates:
+        subject = "Error parsing GWR advance ticket booking dates"
+        body = new_html
+        agenda.mail.send_mail(config, subject, body)
+        return
+
     assert new_dates
     assert list(new_dates.keys()) == ["Weekdays", "Saturdays", "Sundays"]