--- a/mercurial/revlogutils/docket.py	Sun May 30 22:12:48 2021 +0200
+++ b/mercurial/revlogutils/docket.py	Tue May 25 16:46:32 2021 -0700
@@ -95,7 +95,9 @@
 # * 8 bytes: size of data
 # * 8 bytes: pending size of data
 # * 1 bytes: default compression header
-S_HEADER = struct.Struct(constants.INDEX_HEADER.format + 'BBLLLLc')
+S_HEADER = struct.Struct(
+    pycompat.sysstr(constants.INDEX_HEADER.format) + 'BBLLLLc'
+)
 
 
 class RevlogDocket(object):