diff -r 828f44cdfee3 -r fa4d333cac58 mercurial/branchmap.py
--- a/mercurial/branchmap.py	Sat Dec 23 15:11:13 2017 +0530
+++ b/mercurial/branchmap.py	Tue Feb 13 08:41:42 2018 -0500
@@ -377,7 +377,7 @@
                 self._rbcrevs[:] = data
             except (IOError, OSError) as inst:
                 repo.ui.debug("couldn't read revision branch cache: %s\n" %
-                              inst)
+                              pycompat.bytestr(inst))
         # remember number of good records on disk
         self._rbcrevslen = min(len(self._rbcrevs) // _rbcrecsize,
                                len(repo.changelog))