diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index 2adda9c..6bbece6 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -148,7 +148,7 @@ public static function check() {
     mb_language('uni');
 
     // Check for mbstring extension.
-    if (!function_exists('mb_strlen')) {
+    if (!extension_loaded('mbstring')) {
       return 'mb_strlen';
     }