diff --git a/qutil/io.py b/qutil/io.py
index a68bf93a1b55a4abfc902e442d37475ed9d864ec..210a893d55335aa17b8b00c9804b4a76447ea418 100644
--- a/qutil/io.py
+++ b/qutil/io.py
@@ -15,6 +15,7 @@ import warnings
 from concurrent.futures import Future
 from contextlib import contextmanager
 from threading import Thread, Lock
+from typing import Union
 from unittest import mock
 
 import numpy as np