diff --git a/calendar/export_execute.php b/calendar/export_execute.php
index 6d07865ec98..e358c9e7290 100644
--- a/calendar/export_execute.php
+++ b/calendar/export_execute.php
@@ -37,7 +37,7 @@ if(!empty($what) && !empty($time)) {
             foreach ($courses as $course) {
                 $course_groups = groups_get_all_groups($course->id, $user->id);
                 if ($course_groups) {
-                    $groups = $groups + array_keys($course_groups);
+                    $groups = array_merge($groups, array_keys($course_groups));
                 }
             }
             if (empty($groups)) {