diff --git a/dav_events/forms/events.py b/dav_events/forms/events.py
index 4df6d44..0182ccb 100644
--- a/dav_events/forms/events.py
+++ b/dav_events/forms/events.py
@@ -618,7 +618,7 @@ class RegistrationForm(EventCreateForm):
 
     def get_initial_for_field(self, field, field_name):
         value = super(RegistrationForm, self).get_initial_for_field(field, field_name)
-        if field_name == 'registration_howto':
+        if field_name == 'registration_howto' and value is not None:
             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')}