diff --git a/swh/loader/package/nixguix/loader.py b/swh/loader/package/nixguix/loader.py
--- a/swh/loader/package/nixguix/loader.py
+++ b/swh/loader/package/nixguix/loader.py
@@ -135,7 +135,10 @@
             -> Optional[bytes]:
 
         for rev_id, known_artifact in known_artifacts.items():
-            known_integrity = known_artifact['extrinsic']['raw']['integrity']
+            raw = known_artifact['extrinsic']['raw']
+            if 'integrity' not in raw:
+                return None  # old snapshot format
+            known_integrity = raw['integrity']
             if artifact_metadata['integrity'] == known_integrity:
                 return rev_id
         return None