--- a/mercurial/revlog.py	Wed Mar 17 19:06:00 2021 +0100
+++ b/mercurial/revlog.py	Thu Mar 18 18:57:21 2021 -0400
@@ -3250,7 +3250,7 @@
                     # rewriting entries that already have sidedata is not
                     # supported yet, because it introduces garbage data in the
                     # revlog.
-                    msg = "Rewriting existing sidedata is not supported yet"
+                    msg = b"Rewriting existing sidedata is not supported yet"
                     raise error.Abort(msg)
                 entry = entry[:8]
                 entry += (current_offset, len(serialized_sidedata))