diff --git a/falcon/testing/client.py b/falcon/testing/client.py
index 004af27d9..6954ec197 100644
--- a/falcon/testing/client.py
+++ b/falcon/testing/client.py
@@ -463,7 +463,7 @@ async def finalize(self):
 
 
 _HeadersType = Union[None, Mapping[str, str], Iterable[Tuple[str, str]]]
-_AnyApp = Union[WSGI, ASGI]
+_AnyApp = Union['WSGI', 'ASGI']
 _AnyResult = Union[Result, StreamedResult]