diff -r 2f812b0d1936 -r e88fdec9cb9e mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Mon Jun 26 23:57:49 2017 +0530
+++ b/mercurial/debugcommands.py	Mon Jun 26 23:58:27 2017 +0530
@@ -1884,7 +1884,7 @@
     def fmtchunktype(chunktype):
         if chunktype == 'empty':
             return '    %s     : ' % chunktype
-        elif chunktype in string.ascii_letters:
+        elif chunktype in pycompat.bytestr(string.ascii_letters):
             return '    0x%s (%s)  : ' % (hex(chunktype), chunktype)
         else:
             return '    0x%s      : ' % hex(chunktype)