invalid course format "site" when visiting recent activites page

We have used something like

$sectiontitle = get_string('sectionname', 'format_'.$COURSE->format);

in course/recent_form.php
This works all good as long as we are visiting the recent activity page for a specific course, but when moodle/course/recent.php?id=1 (all recent activities page) it generates following error

invalid get_string() identifier: 'sectionname' or component 'format_site'. Perhaps you are missing $string['sectionname'] = ''; in moodle/course/format/site/lang/en/format_site.php?

Thanks