diff --git a/swh/core/db/db_utils.py b/swh/core/db/db_utils.py
--- a/swh/core/db/db_utils.py
+++ b/swh/core/db/db_utils.py
@@ -366,7 +366,9 @@
             query = "select swh_get_dbflavor()"
             try:
                 c.execute(query)
-                return c.fetchone()[0]
+                result = c.fetchone()
+                assert result is not None  # to keep mypy happy
+                return result[0]
             except psycopg2.errors.UndefinedFunction:
                 # function not found: no flavor
                 return None