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
@@ -36,7 +36,7 @@
     if not extra_headers and metadata and "extra_headers" in metadata:
         extra_headers = db_revision["metadata"].pop("extra_headers")
     db_revision["metadata"] = json.dumps(
-        dict(db_revision["metadata"] if db_revision["metadata"] is not None else None)
+        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