diff --git a/src/allmydata/storage/immutable_schema.py b/src/allmydata/storage/immutable_schema.py
index fc823507a..6ac49f6f1 100644
--- a/src/allmydata/storage/immutable_schema.py
+++ b/src/allmydata/storage/immutable_schema.py
@@ -13,6 +13,11 @@ if PY2:
 
 import struct
 
+try:
+    from typing import Union
+except ImportError:
+    pass
+
 import attr
 
 from nacl.hash import blake2b