diff --git a/src/Fetch/Message.php b/src/Fetch/Message.php
index e382678..b679bac 100755
--- a/src/Fetch/Message.php
+++ b/src/Fetch/Message.php
@@ -523,7 +523,7 @@ protected function processStructure($structure, $partIdentifier = null)
             if (!empty($parameters['charset']) && $parameters['charset'] !== self::$charset) {
                 $mb_converted = false;
                 if (function_exists('mb_convert_encoding')) {
-                    if (!in_array($parameters['charset'], mb_list_encodings())) {
+                    if (!in_array(strtoupper($parameters['charset']), array_map('strtoupper', mb_list_encodings()))) {
                         if ($structure->encoding === 0) {
                             $parameters['charset'] = 'US-ASCII';
                         } else {