diff --git a/src/klein/test/typing_app.py b/src/klein/test/typing_app.py
index 63879e9e6..5ba0ada8a 100644
--- a/src/klein/test/typing_app.py
+++ b/src/klein/test/typing_app.py
@@ -2,7 +2,11 @@
 This file contains code that should validate with type checking if type hints
 are correct.
 
-The code is not executed, it is here just to be checked by mypy.
+The code is not executed. Instead, the code should simply validate when checked
+with mypy, or fail and require a `type: ignore` comment.
+
+Because those comments produce an error if mypy thinks they are unnecessary,
+they service to confirm that code that should fail to verify does so.
 """
 
 from twisted.internet.defer import succeed
@@ -11,11 +15,14 @@
 from twisted.web.template import Element, Tag
 
 from klein import Klein, KleinRenderable
+from klein._app import KleinSynchronousRenderable
 
 
 class Application:
     router = Klein()
 
+    # Ensure that various return object types for a route are valid.
+
     @router.route("/object")
     def returnsObject(self, request: IRequest) -> KleinRenderable:
         return object()  # type: ignore[return-value]
@@ -44,6 +51,9 @@ def returnsTag(self, request: IRequest) -> KleinRenderable:
     def returnsNone(self, request: IRequest) -> KleinRenderable:
         return None
 
+    # Ensure that same return object types for a route are valid when wrapped
+    # in a Deferred object.
+
     @router.route("/deferred-object")
     def returnsDeferredObject(self, request: IRequest) -> KleinRenderable:
         return succeed(object())  # type: ignore[arg-type]
@@ -71,3 +81,48 @@ def returnsDeferredTag(self, request: IRequest) -> KleinRenderable:
     @router.route("/deferred-none")
     def returnsDeferredNone(self, request: IRequest) -> KleinRenderable:
         return succeed(None)
+
+    # Ensure that same return object types for a route are valid in async
+    # methods.
+
+    # @router.route("/async-object")
+    # async def asyncReturnsObject(
+    #     self, request: IRequest
+    # ) -> KleinSynchronousRenderable:
+    #     return object()  # type: ignore[arg-type]
+
+    @router.route("/async-str")
+    async def asyncReturnsStr(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return ""
+
+    @router.route("/async-bytes")
+    async def asyncReturnsBytes(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return b""
+
+    @router.route("/async-iresource")
+    async def asyncReturnsIResource(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return Resource()
+
+    @router.route("/async-irenderable")
+    async def asyncReturnsIRenderable(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return Element()
+
+    @router.route("/async-tag")
+    async def asyncReturnsTag(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return Tag("")
+
+    @router.route("/async-none")
+    async def asyncReturnsNone(
+        self, request: IRequest
+    ) -> KleinSynchronousRenderable:
+        return None