diff --git a/lib/configonlylib.php b/lib/configonlylib.php
index 670d69b8ede..df542688628 100644
--- a/lib/configonlylib.php
+++ b/lib/configonlylib.php
@@ -104,7 +104,7 @@ function min_fix_utf8($value) {
     if ($buggyiconv) {
         if (function_exists('mb_convert_encoding')) {
             $subst = mb_substitute_character();
-            mb_substitute_character('');
+            mb_substitute_character('none');
             $result = mb_convert_encoding($value, 'utf-8', 'utf-8');
             mb_substitute_character($subst);
 
diff --git a/lib/moodlelib.php b/lib/moodlelib.php
index 87f44e5ec12..051d0c4a716 100644
--- a/lib/moodlelib.php
+++ b/lib/moodlelib.php
@@ -1284,7 +1284,7 @@ function fix_utf8($value) {
         if ($buggyiconv) {
             if (function_exists('mb_convert_encoding')) {
                 $subst = mb_substitute_character();
-                mb_substitute_character('');
+                mb_substitute_character('none');
                 $result = mb_convert_encoding($value, 'utf-8', 'utf-8');
                 mb_substitute_character($subst);