Index: src/wp-includes/class-IXR.php
===================================================================
--- src/wp-includes/class-IXR.php	(revision 32517)
+++ src/wp-includes/class-IXR.php	(working copy)
@@ -508,7 +508,9 @@
             $xml = '<?xml version="1.0" encoding="'.$charset.'"?>'."\n".$xml;
         else
             $xml = '<?xml version="1.0"?>'."\n".$xml;
+        mbstring_binary_safe_encoding();
         $length = strlen($xml);
+        reset_mbstring_encoding();
         header('Connection: close');
         header('Content-Length: '.$length);
         if ($charset)