diff --git a/README.md b/README.md
index 22e9d21..074f8c6 100644
--- a/README.md
+++ b/README.md
@@ -74,7 +74,7 @@ The current `StreamMessage` is resolvable within DI via the pre-built `faststrea
 ## API
 
 - `setup_di(app, container)` — stores the container in the app context and registers startup/shutdown lifecycle hooks: on startup it reopens the container and adds the DI middleware to every broker of the app (including one added via `app.add_broker` after `setup_di`), after shutdown it closes the container
-- `FromDI(dependency, *, use_cache=True, cast=False)` — FastStream `Depends` that resolves a provider (or type) from the request container
+- `FromDI(dependency, *, use_cache=True, cast=False)` — FastStream `Depends` that resolves a provider (or type) from the request container. Raises `RuntimeError` naming `setup_di` when a message reaches it without the middleware installed
 - `fetch_di_container(app)` — returns the root container from the app context
 - `faststream_message_provider` — `ContextProvider` for the current `faststream.StreamMessage`
 
diff --git a/modern_di_faststream/main.py b/modern_di_faststream/main.py
index cab0863..3696394 100644
--- a/modern_di_faststream/main.py
+++ b/modern_di_faststream/main.py
@@ -19,6 +19,12 @@
 _ROOT_CONTAINER_KEY = "di_container"
 _REQUEST_CONTAINER_KEY = "request_container"
 
+_MISSING_REQUEST_CONTAINER = (
+    "No request container for this message, so the DI middleware did not run for it. "
+    "Call setup_di(app, container) on the app that owns this broker and start the app; "
+    "in tests, pair the test broker with TestApp(app) in the same `async with`."
+)
+
 
 class _DIMiddlewareFactory:
     __slots__ = ("di_container",)
@@ -89,7 +95,9 @@ class Dependency(typing.Generic[T_co]):
     marker: integrations.Marker[T_co]
 
     async def __call__(self, context: faststream.ContextRepo) -> T_co:
-        request_container: Container = context.get(_REQUEST_CONTAINER_KEY)
+        request_container: Container | None = context.get(_REQUEST_CONTAINER_KEY)
+        if request_container is None:
+            raise RuntimeError(_MISSING_REQUEST_CONTAINER)
         return self.marker.resolve(request_container)
 
 
diff --git a/tests/test_faststream_di.py b/tests/test_faststream_di.py
index 1de1797..49ff99a 100644
--- a/tests/test_faststream_di.py
+++ b/tests/test_faststream_di.py
@@ -111,3 +111,35 @@ async def test_middleware_is_installed_once_per_broker_across_restarts() -> None
     for broker in (first, second):
         installed = [m for m in broker.config.broker_middlewares if isinstance(m, _DIMiddlewareFactory)]
         assert len(installed) == 1
+
+
+def _subscribe_with_from_di(broker: NatsBroker, subject: str) -> None:
+    @broker.subscriber(subject)
+    async def subscriber(instance: typing.Annotated[SimpleCreator, FromDI(Dependencies.app_factory)]) -> None:
+        """Never reached: resolving ``instance`` fails first."""
+
+
+async def test_from_di_without_setup_di_names_the_missing_step() -> None:
+    """INVARIANT: ``FromDI`` on a message the DI middleware never saw fails by naming ``setup_di``.
+
+    Broken by handing ``ContextRepo.get``'s ``None`` straight to the marker, which surfaces as
+    modern-di's ``AttributeError: 'NoneType' object has no attribute 'resolve_dependency'`` and
+    points at nothing in this package.
+    """
+    broker = NatsBroker()
+    app_ = faststream.FastStream(broker)
+    _subscribe_with_from_di(broker, TEST_SUBJECT)
+
+    async with TestNatsBroker(broker) as br, TestApp(app_):
+        with pytest.raises(RuntimeError, match="setup_di"):
+            await br.publish(None, TEST_SUBJECT)
+
+
+async def test_from_di_on_an_app_never_started_names_the_missing_step(app: faststream.FastStream) -> None:
+    """The middleware is installed on startup, so a test broker entered without ``TestApp`` has none."""
+    broker = typing.cast(NatsBroker, app.broker)
+    _subscribe_with_from_di(broker, TEST_SUBJECT)
+
+    async with TestNatsBroker(broker) as br:
+        with pytest.raises(RuntimeError, match="TestApp"):
+            await br.publish(None, TEST_SUBJECT)