--- a/mercurial/obsolete.py	Tue Aug 01 17:53:48 2017 +0200
+++ b/mercurial/obsolete.py	Mon Jul 24 10:21:23 2017 -0400
@@ -314,7 +314,7 @@
 _fm1parentshift = 14
 _fm1parentmask = (_fm1parentnone << _fm1parentshift)
 _fm1metapair = 'BB'
-_fm1metapairsize = _calcsize('BB')
+_fm1metapairsize = _calcsize(_fm1metapair)
 
 def _fm1purereadmarkers(data, off, stop):
     # make some global constants local for performance