diff --git a/swh/storage/backfill.py b/swh/storage/backfill.py
--- a/swh/storage/backfill.py
+++ b/swh/storage/backfill.py
@@ -16,7 +16,7 @@
 """
 
 import logging
-from typing import Any, Callable, Dict, Optional
+from typing import Any, Callable, Dict, Iterable, Optional, Tuple, TypeVar
 
 from swh.core.db import BaseDb
 from swh.model.model import (
@@ -446,7 +446,11 @@
             yield start, start + block_size
 
 
-RANGE_GENERATORS = {
+T = TypeVar("T", bytes, int)
+
+RANGE_GENERATORS: Dict[
+    str, Callable[[T, T], Iterable[Tuple[Optional[T], Optional[T]]]]
+] = {
     "content": lambda start, end: byte_ranges(24, start, end),
     "skipped_content": lambda start, end: [(None, None)],
     "directory": lambda start, end: byte_ranges(24, start, end),