diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -179,7 +179,7 @@
         status=sampled_from(["ongoing", "full", "partial"]),
         date=aware_datetimes(),
         snapshot=optional(sha1_git()),
-        metadata=one_of(none(), metadata_dicts()),
+        metadata=optional(metadata_dicts()),
     )
 
 
@@ -191,10 +191,10 @@
 def releases_d(draw):
     target_type = sampled_from([x.value for x in ObjectType])
     name = binary()
-    message = binary()
+    message = optional(binary())
     synthetic = booleans()
     target = sha1_git()
-    metadata = one_of(none(), revision_metadata())
+    metadata = optional(revision_metadata())
 
     return draw(
         one_of(
@@ -234,7 +234,7 @@
 def revisions_d():
     return builds(
         dict,
-        message=binary(),
+        message=optional(binary()),
         synthetic=booleans(),
         author=persons_d(),
         committer=persons_d(),
@@ -243,7 +243,7 @@
         parents=tuples(sha1_git()),
         directory=sha1_git(),
         type=sampled_from([x.value for x in RevisionType]),
-        metadata=one_of(none(), revision_metadata()),
+        metadata=optional(revision_metadata()),
     )
     # TODO: metadata['extra_headers'] can have binary keys and values
 
@@ -350,7 +350,7 @@
     branches = draw(
         dictionaries(
             keys=branch_names(),
-            values=one_of(none(), branch_targets_d(only_objects=only_objects)),
+            values=optional(branch_targets_d(only_objects=only_objects)),
             min_size=min_size,
             max_size=max_size,
         )
diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -407,7 +407,7 @@
 
 @attr.s(frozen=True)
 class Revision(BaseModel, HashableObject):
-    message = attr.ib(type=bytes, validator=type_validator())
+    message = attr.ib(type=Optional[bytes], validator=type_validator())
     author = attr.ib(type=Person, validator=type_validator())
     committer = attr.ib(type=Person, validator=type_validator())
     date = attr.ib(type=Optional[TimestampWithTimezone], validator=type_validator())