diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -958,7 +958,7 @@
                     self.ferr.flush()
         except IOError as inst:
             if inst.errno not in (errno.EPIPE, errno.EIO, errno.EBADF):
-                raise error.StdioError(inst)
+                raise error.StdioError(pycompat.bytestr(inst))
 
     def flush(self):
         # opencode timeblockedsection because this is a critical path