diff --git a/xarray/backends/api.py b/xarray/backends/api.py
index 13162a48a69..93819d35bb8 100644
--- a/xarray/backends/api.py
+++ b/xarray/backends/api.py
@@ -2,6 +2,7 @@
 
 import os
 from collections.abc import Hashable, Iterable, Mapping, MutableMapping, Sequence
+from typing import Optional
 from functools import partial
 from io import BytesIO
 from numbers import Number