diff --git a/swh/loader/pypi/client.py b/swh/loader/pypi/client.py
--- a/swh/loader/pypi/client.py
+++ b/swh/loader/pypi/client.py
@@ -359,7 +359,8 @@
         """
         return info(self._data(release_name))
 
-    def _filter_release_artifacts(self, version, releases, known_artifacts):
+    def _filter_release_artifacts(self, version, releases,
+                                  known_artifacts=None):
         """Filter not already known sdist (source distribution) release.
 
         There can be multiple 'package_type' (sdist, bdist_egg,
@@ -381,6 +382,10 @@
             return []
         if not isinstance(releases, list):
             releases = [releases]
+
+        if not known_artifacts:
+            known_artifacts = set()
+
         for artifact in releases:
             name = artifact['filename']
             sha256 = artifact['digests']['sha256']