diff -urN ../z-push-2.6.2+0.orig/src/include/mimeDecode.php ./src/include/mimeDecode.php
--- ../z-push-2.6.2+0.orig/src/include/mimeDecode.php	2021-06-25 11:55:30.000000000 +0300
+++ ./src/include/mimeDecode.php	2023-03-18 01:54:42.028409000 +0200
@@ -861,7 +861,7 @@
                 $input = base64_decode($input);
                 break;
         }
-        if ($detectCharset && strtolower($charset) != $this->_charset) {
+        if ($detectCharset && $charset && strtolower($charset) != $this->_charset) {
             $conv = @iconv($charset, $this->_charset, $input);
             $input = ($conv === false) ? $input : $conv;
         }