diff --git a/swh/storage/cassandra/converters.py b/swh/storage/cassandra/converters.py
--- a/swh/storage/cassandra/converters.py
+++ b/swh/storage/cassandra/converters.py
@@ -35,7 +35,9 @@
     extra_headers = revision.extra_headers
     if not extra_headers and metadata and "extra_headers" in metadata:
         extra_headers = db_revision["metadata"].pop("extra_headers")
-    db_revision["metadata"] = json.dumps(db_revision["metadata"])
+    db_revision["metadata"] = json.dumps(
+        dict(db_revision["metadata"] if db_revision["metadata"] is not None else None)
+    )
     db_revision["extra_headers"] = extra_headers
     db_revision["type"] = db_revision["type"].value
     return db_revision
diff --git a/swh/storage/cassandra/cql.py b/swh/storage/cassandra/cql.py
--- a/swh/storage/cassandra/cql.py
+++ b/swh/storage/cassandra/cql.py
@@ -779,7 +779,9 @@
         assert self._origin_visit_status_keys[-1] == "metadata"
         keys = self._origin_visit_status_keys
 
-        metadata = json.dumps(visit_update.metadata)
+        metadata = json.dumps(
+            dict(visit_update.metadata) if visit_update.metadata is not None else None
+        )
         self._execute_with_retries(
             statement, [getattr(visit_update, key) for key in keys[:-1]] + [metadata]
         )
diff --git a/swh/storage/db.py b/swh/storage/db.py
--- a/swh/storage/db.py
+++ b/swh/storage/db.py
@@ -9,11 +9,15 @@
 from typing import Any, Dict, Iterable, List, Optional, Tuple, Union
 
 from swh.core.db import BaseDb
-from swh.core.db.db_utils import stored_procedure, jsonize
+from swh.core.db.db_utils import stored_procedure, jsonize as _jsonize
 from swh.core.db.db_utils import execute_values_generator
 from swh.model.model import OriginVisit, OriginVisitStatus, SHA1_SIZE
 
 
+def jsonize(d):
+    return _jsonize(dict(d) if d is not None else None)
+
+
 class Db(BaseDb):
     """Proxy to the SWH DB, with wrappers around stored procedures