diff --git a/cohort/classes/local/entities/cohort.php b/cohort/classes/local/entities/cohort.php
index 8a01ded522f..f70c644dd21 100644
--- a/cohort/classes/local/entities/cohort.php
+++ b/cohort/classes/local/entities/cohort.php
@@ -132,10 +132,14 @@ class cohort extends base {
             ->add_joins($this->get_joins())
             ->set_type(column::TYPE_TEXT)
             ->add_fields("{$tablealias}.description, {$tablealias}.descriptionformat, {$tablealias}.id, {$tablealias}.contextid")
-            ->add_callback(static function(string $description, stdClass $cohort): string {
+            ->add_callback(static function(?string $description, stdClass $cohort): string {
                 global $CFG;
                 require_once("{$CFG->libdir}/filelib.php");
 
+                if ($description === null) {
+                    return '';
+                }
+
                 $description = file_rewrite_pluginfile_urls($description, 'pluginfile.php', $cohort->contextid, 'cohort',
                     'description', $cohort->id);
 
diff --git a/course/classes/local/entities/course_category.php b/course/classes/local/entities/course_category.php
index 2ed60ffa63e..6c923b0058a 100644
--- a/course/classes/local/entities/course_category.php
+++ b/course/classes/local/entities/course_category.php
@@ -141,10 +141,14 @@ class course_category extends base {
                  AND {$tablealiascontext}.contextlevel = " . CONTEXT_COURSECAT)
             ->set_type(column::TYPE_TEXT)
             ->add_fields("{$tablealias}.description, {$tablealias}.descriptionformat, {$tablealiascontext}.id AS contextid")
-            ->add_callback(static function(string $description, stdClass $category): string {
+            ->add_callback(static function(?string $description, stdClass $category): string {
                 global $CFG;
                 require_once("{$CFG->libdir}/filelib.php");
 
+                if ($description === null) {
+                    return '';
+                }
+
                 $description = file_rewrite_pluginfile_urls($description, 'pluginfile.php', $category->contextid, 'coursecat',
                     'description', null);