diff --git a/swh/loader/pypi/loader.py b/swh/loader/pypi/loader.py
--- a/swh/loader/pypi/loader.py
+++ b/swh/loader/pypi/loader.py
@@ -110,10 +110,15 @@
         if not last_snapshot or 'branches' not in last_snapshot:
             return {}
 
-        revs = [rev['target'] for rev in last_snapshot['branches'].values()]
+        # retrieve only revisions (e.g the alias we do not want here)
+        revs = [rev['target']
+                for rev in last_snapshot['branches'].values()
+                if rev['target_type'] == 'revision']
         known_revisions = self.storage.revision_get(revs)
         ret = {}
         for revision in known_revisions:
+            if not revision:  # revision_get can return None
+                continue
             if 'original_artifact' in revision['metadata']:
                 artifact = revision['metadata']['original_artifact']
                 ret[artifact['filename'], artifact['sha256']] = revision['id']