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
@@ -2896,7 +2896,7 @@
                     self.origin_metadata2['metadata'])
         self.storage.origin_metadata_add(
                     origin_id,
-                    self.origin_metadata2['discovery_date'],
+                    str(self.origin_metadata2['discovery_date']),
                     provider['id'],
                     tool['id'],
                     self.origin_metadata2['metadata'])