diff --git a/addons/calendar/models/calendar.py b/addons/calendar/models/calendar.py
index 315f2f5d411..bd65166fd65 100644
--- a/addons/calendar/models/calendar.py
+++ b/addons/calendar/models/calendar.py
@@ -160,7 +160,7 @@ class Attendee(models.Model):
         mails_to_send = self.env['mail.mail']
         for attendee in self:
             if attendee.email or attendee.partner_id.email:
-                ics_file = ics_files[attendee.event_id.id]
+                ics_file = ics_files.get(attendee.event_id.id)
                 mail_id = invitation_template.send_mail(attendee.id)
 
                 vals = {}
@@ -850,6 +850,7 @@ class Meeting(models.Model):
             # FIXME: why isn't this in CalDAV?
             import vobject
         except ImportError:
+            _logger.warning("The `vobject` Python module is not installed, so iCal file generation is unavailable. Use 'pip install vobject' to install it")
             return result
 
         for meeting in self: