--- a/mercurial/tags.py	Sat Feb 24 16:15:16 2018 +0530
+++ b/mercurial/tags.py	Sat Feb 24 16:20:15 2018 +0530
@@ -783,6 +783,6 @@
         except (IOError, OSError) as inst:
             repo.ui.log('tagscache',
                         "couldn't write cache/%s: %s\n" % (
-                        _fnodescachefile, inst))
+                        _fnodescachefile, util.forcebytestr(inst)))
         finally:
             lock.release()