diff --git a/packages/express/src/index.test.ts b/packages/express/src/index.test.ts
new file mode 100644
index 000000000..ee8a3d051
--- /dev/null
+++ b/packages/express/src/index.test.ts
@@ -0,0 +1,41 @@
+import type { Federation } from "@fedify/fedify";
+import type {
+  NextFunction,
+  Request as ERequest,
+  Response as EResponse,
+} from "express";
+import { strict as assert } from "node:assert";
+import { test } from "node:test";
+
+import { integrateFederation } from "./index.ts";
+
+test("integrateFederation delegates not-found requests to Express", async () => {
+  let nextCalls = 0;
+  const federation = {
+    fetch: (
+      _request: Request,
+      options: { onNotFound: () => Response },
+    ) => {
+      options.onNotFound();
+      return new Response("unused", { status: 404 });
+    },
+  } as unknown as Federation<void>;
+
+  const request = {
+    protocol: "https",
+    host: "example.com",
+    url: "/not-a-federation-route",
+    method: "GET",
+    headers: {},
+  } as ERequest;
+  const response = {} as EResponse;
+  const next: NextFunction = () => {
+    nextCalls++;
+  };
+
+  const middleware = integrateFederation(federation, () => undefined);
+  middleware(request, response, next);
+  await new Promise<void>((resolve) => setImmediate(resolve));
+
+  assert.equal(nextCalls, 1);
+});