diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -371,7 +371,7 @@
         type=bytes,
         validator=type_validator())
     message = attr.ib(
-        type=bytes,
+        type=Optional[bytes],
         validator=type_validator())
     target = attr.ib(
         type=Optional[Sha1Git],