diff --git a/falcon/_typing.py b/falcon/_typing.py
index d82a5bac5..c8f27c877 100644
--- a/falcon/_typing.py
+++ b/falcon/_typing.py
@@ -92,12 +92,14 @@ async def __call__(
 
 
 class SinkCallable(Protocol):
-    def __call__(self, req: Request, resp: Response, **kwargs: str) -> None: ...
+    def __call__(
+        self, req: Request, resp: Response, **kwargs: Optional[str]
+    ) -> None: ...
 
 
 class AsgiSinkCallable(Protocol):
     async def __call__(
-        self, req: AsgiRequest, resp: AsgiResponse, **kwargs: str
+        self, req: AsgiRequest, resp: AsgiResponse, **kwargs: Optional[str]
     ) -> None: ...