diff --git a/mercurial/httppeer.py b/mercurial/httppeer.py
--- a/mercurial/httppeer.py
+++ b/mercurial/httppeer.py
@@ -278,6 +278,8 @@
         hgargssize = None
 
         for header, value in sorted(req.header_items()):
+            header = pycompat.bytesurl(header)
+            value = pycompat.bytesurl(value)
             if header.startswith('X-hgarg-'):
                 if hgargssize is None:
                     hgargssize = 0