diff --git a/completion/classes/activity_custom_completion.php b/completion/classes/activity_custom_completion.php
index 7c7806a5f7c..2ac83dd11ad 100644
--- a/completion/classes/activity_custom_completion.php
+++ b/completion/classes/activity_custom_completion.php
@@ -96,8 +96,9 @@ abstract class activity_custom_completion {
     public function get_available_custom_rules(): array {
         $rules = static::get_defined_custom_rules();
         $availablerules = [];
+        $customdata = (array)$this->cm->customdata;
         foreach ($rules as $rule) {
-            $customrule = $this->cm->customdata['customcompletionrules'][$rule] ?? false;
+            $customrule = $customdata['customcompletionrules'][$rule] ?? false;
             if (!empty($customrule)) {
                 $availablerules[] = $rule;
             }
diff --git a/lib/completionlib.php b/lib/completionlib.php
index 053be164197..5318a86df09 100644
--- a/lib/completionlib.php
+++ b/lib/completionlib.php
@@ -1107,8 +1107,10 @@ class completion_info {
 
         // Custom activity module completion data.
 
+        // Cast custom data to array before checking for custom completion rules.
+        $customdata = (array)$cm->customdata;
         // Return early if the plugin does not define custom completion rules.
-        if (empty($cm->customdata['customcompletionrules'])) {
+        if (empty($customdata['customcompletionrules'])) {
             return [];
         }
 
@@ -1120,7 +1122,7 @@ class completion_info {
 
         /** @var activity_custom_completion $customcmcompletion */
         $customcmcompletion = new $cmcompletionclass($cm, $userid);
-        foreach ($cm->customdata['customcompletionrules'] as $rule => $enabled) {
+        foreach ($customdata['customcompletionrules'] as $rule => $enabled) {
             if (!$enabled) {
                 // Skip inactive completion rules.
                 continue;