Correction again :)

app/views/events/show.ics.haml

@@ -1,13 +1,13 @@
 :ruby
   # Create a calendar with an event (standard method)
   cal = Icalendar::Calendar.new
-  tzid = event.region.tzid
+  tzid = @event.region.tzid
   use_timezone = true
   if cal.timezones.none? { |t| tzid == t.tzid }
     # Only add this zone once
     begin
       tz = TZInfo::Timezone.get tzid
-      timezone = tz.ical_timezone event.start_time
+      timezone = tz.ical_timezone @event.start_time
       cal.add_timezone timezone
     rescue TZInfo::InvalidTimezoneIdentifier
       # No need to add this tz