--- a/mercurial/debugcommands.py	Thu Mar 01 23:52:30 2018 +0530
+++ b/mercurial/debugcommands.py	Thu Mar 01 23:54:52 2018 +0530
@@ -1591,7 +1591,8 @@
                                      metadata=metadata, ui=ui)
                 tr.close()
             except ValueError as exc:
-                raise error.Abort(_('bad obsmarker input: %s') % exc)
+                raise error.Abort(_('bad obsmarker input: %s') %
+                                  pycompat.bytestr(exc))
             finally:
                 tr.release()
         finally: