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
@@ -42,8 +42,11 @@
         self.sources = s['sources']
         self.provider_url = url
 
-        self._integrityByUrl = {s['urls'][0]: s['integrity']
-                                for s in self.sources}
+        self._integrityByUrl = {
+            s['urls'][0]: s['integrity']
+            for s in self.sources
+            if 'integrity' in s
+        }
 
         # The revision used to create the sources.json file. For Nix,
         # this revision belongs to the github.com/nixos/nixpkgs
@@ -77,7 +80,10 @@
             -> 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