diff --git a/dav_events/forms/events.py b/dav_events/forms/events.py
index 0585648..b29d7f6 100644
--- a/dav_events/forms/events.py
+++ b/dav_events/forms/events.py
@@ -654,7 +654,7 @@ class RegistrationForm(EventCreateForm):
             trainer_fullname = u'%s %s' % (self._session_data.get('trainer_firstname'),
                                            self._session_data.get('trainer_familyname'))
             value = value % {'name': trainer_fullname, 'emailaddr': self._session_data.get('trainer_email')}
-        elif field_name in ('min_participants', 'max_participants'):
+        elif field_name in ('min_participants', 'max_participants') and value is None:
             trainer_2 = bool(self._session_data.get('trainer_2_fullname', False))
             trainer_3 = bool(self._session_data.get('trainer_3_fullname', False))
             matrix_key, matrix_config = self._get_matrix_config(self._session_data)