diff --git a/src/borg/upgrade.py b/src/borg/upgrade.py
index 2f88d791f..1dfef64ec 100644
--- a/src/borg/upgrade.py
+++ b/src/borg/upgrade.py
@@ -73,10 +73,10 @@ def upgrade_compressed_chunk(self, *, chunk):
         def upgrade_zlib_and_level(chunk):
             if ZLIB_legacy.detect(chunk):
                 ctype = ZLIB.ID
-                chunk = ctype + level + chunk  # get rid of the attic legacy: prepend separate type/level bytes
+                chunk = ctype + level + bytes(chunk)  # get rid of the attic legacy: prepend separate type/level bytes
             else:
-                ctype = chunk[0:1]
-                chunk = ctype + level + chunk[2:]  # keep type same, but set level
+                ctype = bytes(chunk[0:1])
+                chunk = ctype + level + bytes(chunk[2:])  # keep type same, but set level
             return chunk
 
         ctype = chunk[0:1]