diff --git a/swh/provenance/postgresql/archive.py b/swh/provenance/postgresql/archive.py
--- a/swh/provenance/postgresql/archive.py
+++ b/swh/provenance/postgresql/archive.py
@@ -3,7 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
-from typing import Any, Dict, Iterable, List, Tuple
+from typing import Any, Dict, Iterable, List, Tuple, cast
 
 import psycopg2.extensions
 
@@ -112,7 +112,7 @@
                 """,
                 (revision_id,),
             )
-            return cursor.fetchall()
+            return cast(List[Tuple[Sha1Git, Sha1Git]], cursor.fetchall())
 
     @statsd.timed(metric=ARCHIVE_DURATION_METRIC, tags={"method": "snapshot_get_heads"})
     def snapshot_get_heads(self, id: Sha1Git) -> Iterable[Sha1Git]:
diff --git a/swh/provenance/postgresql/provenance.py b/swh/provenance/postgresql/provenance.py
--- a/swh/provenance/postgresql/provenance.py
+++ b/swh/provenance/postgresql/provenance.py
@@ -90,7 +90,9 @@
         if self._flavor is None:
             with self.transaction(readonly=True) as cursor:
                 cursor.execute("SELECT swh_get_dbflavor() AS flavor")
-                self._flavor = cursor.fetchone()["flavor"]
+                flavor = cursor.fetchone()
+                assert flavor  # please mypy
+                self._flavor = flavor["flavor"]
         assert self._flavor is not None
         return self._flavor