diff --git a/src/wp-includes/sodium_compat/src/Core/Util.php b/src/wp-includes/sodium_compat/src/Core/Util.php
index a5808b191d..aa93dfd044 100644
--- a/src/wp-includes/sodium_compat/src/Core/Util.php
+++ b/src/wp-includes/sodium_compat/src/Core/Util.php
@@ -911,6 +911,7 @@ abstract class ParagonIE_Sodium_Core_Util
 
         if ($mbstring === null) {
             $mbstring = extension_loaded('mbstring')
+                && defined('MB_OVERLOAD_STRING')
                 &&
             ((int) (ini_get('mbstring.func_overload')) & MB_OVERLOAD_STRING);
         }