When creating a new Questionnaire activity and setting a 'Close Date' in the 'Timing' section, the following error is produced when clicking 'Save and return to course':

"Fatal error: Class 'calendar_event' not found in /***/mod/questionnaire/locallib.php on line 600"

Using the MOODLE_26_STABLE branch from git://github.com/remotelearner/moodle-mod_questionnaire.git