diff --git a/trytond/wsgi.py b/trytond/wsgi.py
index 01c3bb608f..e6ceb0f086 100644
--- a/trytond/wsgi.py
+++ b/trytond/wsgi.py
@@ -53,7 +53,7 @@ class Base64Converter(BaseConverter):
         return base64.urlsafe_b64decode(value).decode('utf-8')
 
     def to_url(self, value):
-        return base64.urlsafe_b64encode(value.encode('utf-8'))
+        return base64.urlsafe_b64encode(value.encode('utf-8')).decode('ascii')
 
 
 class TrytondWSGI(object):