diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
index d9a813a17a8f674d4403bc840eda0ffbbedfb684..cdd4d1abe031179b67d1ba3a91929665ccc5b5df 100644
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -422,11 +422,11 @@ def snapshots(*, min_size=0, max_size=100, only_objects=False):
 
 
 def metadata_authorities():
-    return builds(MetadataAuthority, url=urls(), metadata=just({}))
+    return builds(MetadataAuthority, url=urls(), metadata=just(None))
 
 
 def metadata_fetchers():
-    return builds(MetadataFetcher, metadata=just({}))
+    return builds(MetadataFetcher, metadata=just(None))
 
 
 def raw_extrinsic_metadata():