--- a/mercurial/url.py	Fri Apr 24 20:00:25 2020 +0200
+++ b/mercurial/url.py	Sun Apr 26 14:29:47 2020 -0400
@@ -225,7 +225,7 @@
 
 def _generic_proxytunnel(self):
     proxyheaders = {
-        x: self.headers[x]
+        pycompat.bytestr(x): pycompat.bytestr(self.headers[x])
         for x in self.headers
         if x.lower().startswith('proxy-')
     }