diff --git a/hgext/convert/subversion.py b/hgext/convert/subversion.py
--- a/hgext/convert/subversion.py
+++ b/hgext/convert/subversion.py
@@ -147,7 +147,7 @@
         # Caller may interrupt the iteration
         pickle.dump(None, fp, protocol)
     except Exception as inst:
-        pickle.dump(str(inst), fp, protocol)
+        pickle.dump(util.forcebytestr(inst), fp, protocol)
     else:
         pickle.dump(None, fp, protocol)
     fp.flush()
diff --git a/hgext/mq.py b/hgext/mq.py
--- a/hgext/mq.py
+++ b/hgext/mq.py
@@ -723,7 +723,8 @@
         try:
             os.unlink(undo)
         except OSError as inst:
-            self.ui.warn(_('error removing undo: %s\n') % str(inst))
+            self.ui.warn(_('error removing undo: %s\n') %
+                         util.forcebytestr(inst))
 
     def backup(self, repo, files, copy=False):
         # backup local changes in --force case
diff --git a/hgext/narrow/narrowbundle2.py b/hgext/narrow/narrowbundle2.py
--- a/hgext/narrow/narrowbundle2.py
+++ b/hgext/narrow/narrowbundle2.py
@@ -449,7 +449,7 @@
         except OSError as e:
             if e.errno != errno.ENOENT:
                 ui.warn(_('error removing %s: %s\n') %
-                        (undovfs.join(undofile), str(e)))
+                        (undovfs.join(undofile), util.forcebytestr(e)))
 
     # Remove partial backup only if there were no exceptions
     vfs.unlink(chgrpfile)
diff --git a/hgext/relink.py b/hgext/relink.py
--- a/hgext/relink.py
+++ b/hgext/relink.py
@@ -187,7 +187,7 @@
             relinked += 1
             savedbytes += sz
         except OSError as inst:
-            ui.warn('%s: %s\n' % (tgt, str(inst)))
+            ui.warn('%s: %s\n' % (tgt, util.forcebytestr(inst)))
 
     ui.progress(_('relinking'), None)