diff --git a/swh/storage/cassandra/storage.py b/swh/storage/cassandra/storage.py
--- a/swh/storage/cassandra/storage.py
+++ b/swh/storage/cassandra/storage.py
@@ -1071,16 +1071,19 @@
         if not self._cql_runner.metadata_fetcher_get(**fetcher):
             raise StorageArgumentException(f"Unknown fetcher {fetcher}")
 
-        self._cql_runner.origin_metadata_add(
-            origin_url,
-            authority["type"],
-            authority["url"],
-            discovery_date,
-            fetcher["name"],
-            fetcher["version"],
-            format,
-            metadata,
-        )
+        try:
+            self._cql_runner.origin_metadata_add(
+                origin_url,
+                authority["type"],
+                authority["url"],
+                discovery_date,
+                fetcher["name"],
+                fetcher["version"],
+                format,
+                metadata,
+            )
+        except TypeError as e:
+            raise StorageArgumentException(*e.args)
 
     def origin_metadata_get(
         self,
diff --git a/swh/storage/db.py b/swh/storage/db.py
--- a/swh/storage/db.py
+++ b/swh/storage/db.py
@@ -1096,8 +1096,7 @@
                     authority_id, fetcher_id, format, metadata)
                     SELECT id, %s, %s, %s, %s, %s FROM origin WHERE url = %s"""
         cur.execute(
-            insert,
-            (discovery_date, authority, fetcher, format, jsonize(metadata), origin),
+            insert, (discovery_date, authority, fetcher, format, metadata, origin),
         )
 
     def origin_metadata_get(
diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -1059,6 +1059,10 @@
             raise StorageArgumentException(
                 "origin_id must be str, not %r" % (origin_url,)
             )
+        if not isinstance(metadata, bytes):
+            raise StorageArgumentException(
+                "metadata must be bytes, not %r" % (metadata,)
+            )
         authority_key = self._metadata_authority_key(authority)
         if authority_key not in self._metadata_authorities:
             raise StorageArgumentException(f"Unknown authority {authority}")
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -1255,9 +1255,18 @@
         )
         if not fetcher_id:
             raise StorageArgumentException(f"Unknown fetcher {fetcher}")
-        db.origin_metadata_add(
-            origin_url, discovery_date, authority_id, fetcher_id, format, metadata, cur
-        )
+        try:
+            db.origin_metadata_add(
+                origin_url,
+                discovery_date,
+                authority_id,
+                fetcher_id,
+                format,
+                metadata,
+                cur,
+            )
+        except psycopg2.ProgrammingError as e:
+            raise StorageArgumentException(*e.args)
         send_metric("origin_metadata:add", count=1, method_name="origin_metadata_add")
 
     @timed
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -3225,6 +3225,21 @@
             )
         )
 
+    def test_origin_metadata_add_dict(self, swh_storage):
+        origin = data.origin
+        fetcher = data.metadata_fetcher
+        authority = data.metadata_authority
+        swh_storage.origin_add([origin])[0]
+
+        swh_storage.metadata_fetcher_add(**fetcher)
+        swh_storage.metadata_authority_add(**authority)
+
+        kwargs = data.origin_metadata.copy()
+        kwargs["metadata"] = {"foo": "bar"}
+
+        with pytest.raises(StorageArgumentException):
+            swh_storage.origin_metadata_add(**kwargs)
+
     def test_origin_metadata_get(self, swh_storage):
         authority = data.metadata_authority
         fetcher = data.metadata_fetcher