diff --git a/capa/features/extractors/base_extractor.py b/capa/features/extractors/base_extractor.py
index b88da5f8..46908fb2 100644
--- a/capa/features/extractors/base_extractor.py
+++ b/capa/features/extractors/base_extractor.py
@@ -12,6 +12,7 @@ import dataclasses
 from typing import Any, Dict, Tuple, Union, Iterator
 from dataclasses import dataclass
 
+# TODO(williballenthin): use typing.TypeAlias directly when Python 3.9 is deprecated
 from typing_extensions import TypeAlias
 
 import capa.features.address