diff --git a/starlette/middleware/__init__.py b/starlette/middleware/__init__.py
index 8e0a54edb..5164e49e0 100644
--- a/starlette/middleware/__init__.py
+++ b/starlette/middleware/__init__.py
@@ -14,7 +14,7 @@
 
 
 class _MiddlewareFactory(Protocol[P]):
-    def __call__(self, app: ASGIApp, *args: P.args, **kwargs: P.kwargs) -> ASGIApp: ...  # pragma: no cover
+    def __call__(self, app: ASGIApp, /, *args: P.args, **kwargs: P.kwargs) -> ASGIApp: ...  # pragma: no cover
 
 
 class Middleware:
diff --git a/tests/test_applications.py b/tests/test_applications.py
index 29c011a29..db2a6050b 100644
--- a/tests/test_applications.py
+++ b/tests/test_applications.py
@@ -3,7 +3,7 @@
 import os
 from contextlib import asynccontextmanager
 from pathlib import Path
-from typing import AsyncGenerator, AsyncIterator, Generator
+from typing import AsyncGenerator, AsyncIterator, Callable, Generator
 
 import anyio.from_thread
 import pytest
@@ -567,9 +567,12 @@ async def _app(scope: Scope, receive: Receive, send: Send) -> None:
 
         return _app
 
+    def get_middleware_factory() -> Callable[[ASGIApp, str], ASGIApp]:
+        return _middleware_factory
+
     app = Starlette()
     app.add_middleware(_middleware_factory, arg="foo")
-    app.add_middleware(_middleware_factory, arg="bar")
+    app.add_middleware(get_middleware_factory(), "bar")
 
     with test_client_factory(app):
         pass