diff --git a/swh/scrubber/db.py b/swh/scrubber/db.py
--- a/swh/scrubber/db.py
+++ b/swh/scrubber/db.py
@@ -74,7 +74,9 @@
                 """,
                 (dataclasses.asdict(datastore)),
             )
-            (id_,) = cur.fetchone()
+            res = cur.fetchone()
+            assert res is not None
+            (id_,) = res
             return id_
 
     def corrupt_object_add(