diff --git a/calendar/classes/local/event/forms/create.php b/calendar/classes/local/event/forms/create.php
index 90ebb358777..7775e45860f 100644
--- a/calendar/classes/local/event/forms/create.php
+++ b/calendar/classes/local/event/forms/create.php
@@ -128,6 +128,7 @@ class create extends \moodleform {
         $eventtype = isset($data['eventtype']) ? $data['eventtype'] : null;
         $coursekey = ($eventtype == 'group') ? 'groupcourseid' : 'courseid';
         $courseid = (!empty($data[$coursekey])) ? $data[$coursekey] : null;
+        $categoryid = (!empty($data['categoryid'])) ? $data['categoryid'] : null;
 
         $eventtypes = $this->_customdata['eventtypes'];
         if (empty($eventtype) || !isset($eventtypes[$eventtype]) || $eventtypes[$eventtype] == false) {
@@ -148,6 +149,10 @@ class create extends \moodleform {
             $errors['courseid'] = get_string('selectacourse');
         }
 
+        if ($eventtype == 'category' && empty($categoryid)) {
+            $errors['categoryid'] = get_string('selectacategory');
+        }
+
         if ($eventtype == 'group' && (!empty($courseid) && empty($data['groupid']))) {
             $errors['groupcourseid'] = get_string('nogroups', 'core_group');
         }
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 876e190d7ae..40efad053fb 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2387,5 +2387,11 @@ function xmldb_main_upgrade($oldversion) {
         upgrade_main_savepoint(true, 2021052500.42);
     }
 
+    if ($oldversion < 2021052500.55) {
+        $DB->delete_records_select('event', "eventtype = 'category' AND categoryid = 0 AND userid <> 0");
+
+        upgrade_main_savepoint(true, 2021052500.55);
+    }
+
     return true;
 }
diff --git a/version.php b/version.php
index 468878bdb2a..1fe0747f146 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2021052500.54;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2021052500.55;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.
 $release  = '4.0dev (Build: 20210130)'; // Human-friendly version name