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
@@ -8,7 +8,7 @@
 import attr
 
 from copy import deepcopy
-from typing import Any, Dict, Tuple
+from typing import cast, Any, Dict, Optional, Tuple
 
 from cassandra.cluster import ResultSet
 
@@ -22,6 +22,7 @@
 )
 from swh.model.hashutil import DEFAULT_ALGORITHMS
 
+from swh.storage.utils import map_optional
 from .common import Row
 
 
@@ -35,7 +36,9 @@
     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(
+        map_optional(dict, cast(Optional[Dict], db_revision["metadata"]))
+    )
     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
@@ -44,6 +44,8 @@
     Origin,
 )
 
+from swh.storage.utils import map_optional
+
 from .common import Row, TOKEN_BEGIN, TOKEN_END, hash_url
 from .schema import CREATE_TABLES_QUERIES, HASH_ALGORITHMS
 from .. import extrinsic_metadata
@@ -779,7 +781,7 @@
         assert self._origin_visit_status_keys[-1] == "metadata"
         keys = self._origin_visit_status_keys
 
-        metadata = json.dumps(visit_update.metadata)
+        metadata = json.dumps(map_optional(dict, visit_update.metadata))
         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
@@ -9,10 +9,16 @@
 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
 
+from .utils import map_optional
+
+
+def jsonize(d):
+    return _jsonize(map_optional(dict, d))
+
 
 class Db(BaseDb):
     """Proxy to the SWH DB, with wrappers around stored procedures
diff --git a/swh/storage/utils.py b/swh/storage/utils.py
--- a/swh/storage/utils.py
+++ b/swh/storage/utils.py
@@ -6,7 +6,7 @@
 import re
 
 from datetime import datetime, timezone
-from typing import Dict, Optional, Tuple
+from typing import Callable, Dict, Optional, Tuple, TypeVar
 
 from swh.model.hashutil import hash_to_bytes, hash_to_hex, DEFAULT_ALGORITHMS
 
@@ -15,6 +15,18 @@
     return datetime.now(tz=timezone.utc)
 
 
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+
+
+def map_optional(f: Callable[[T1], T2], x: Optional[T1]) -> Optional[T2]:
+    """Equivalent to `None if x is None else f(x)`."""
+    if x is None:
+        return None
+    else:
+        return f(x)
+
+
 def _is_power_of_two(n: int) -> bool:
     return n > 0 and n & (n - 1) == 0