diff --git a/mercurial/mail.py b/mercurial/mail.py
--- a/mercurial/mail.py
+++ b/mercurial/mail.py
@@ -187,7 +187,7 @@
 
 def codec2iana(cs):
     ''''''
-    cs = email.charset.Charset(cs).input_charset.lower()
+    cs = pycompat.sysbytes(email.charset.Charset(cs).input_charset.lower())
 
     # "latin1" normalizes to "iso8859-1", standard calls for "iso-8859-1"
     if cs.startswith("iso") and not cs.startswith("iso-"):