diff --git a/mercurial/revlogutils/docket.py b/mercurial/revlogutils/docket.py
--- a/mercurial/revlogutils/docket.py
+++ b/mercurial/revlogutils/docket.py
@@ -93,7 +93,9 @@
 # * 8 bytes: size of data
 # * 8 bytes: pending size of data
 # * 1 bytes: default compression header
-S_HEADER = struct.Struct(constants.INDEX_HEADER.format + 'LLLLc')
+S_HEADER = struct.Struct(
+    pycompat.sysstr(constants.INDEX_HEADER.format) + 'LLLLc'
+)
 
 
 class RevlogDocket(object):