Added encoding to StringHelper::byteSubst() fallback

framework/helpers/BaseStringHelper.php

@@ -40,7 +40,7 @@ class BaseStringHelper
      */
     public static function byteSubstr($string, $start, $length)
     {
-        return mb_substr($string, $start, $length ?: mb_strlen($string), '8bit');
+        return mb_substr($string, $start, $length ?: mb_strlen($string, '8bit'), '8bit');
     }
 
     /**