diff --git a/src/create-with-winter-spec.ts b/src/create-with-winter-spec.ts
index 229a28c..bc1c4f6 100644
--- a/src/create-with-winter-spec.ts
+++ b/src/create-with-winter-spec.ts
@@ -126,6 +126,7 @@ function serializeResponse(
 ): Middleware {
   return async (req, ctx, next) => {
     const rawResponse = await next(req, ctx)
+    assertResponseWasReturned(rawResponse)
 
     const statusCode =
       rawResponse instanceof WinterSpecResponse
@@ -150,6 +151,18 @@ function serializeResponse(
   }
 }
 
+function assertResponseWasReturned(
+  rawResponse: unknown
+): asserts rawResponse is SerializableToResponse | Response {
+  if (rawResponse == null || typeof rawResponse !== "object") {
+    throw new Error(
+      `Use ctx.json({...}) or return a Response instead of returning ${String(
+        rawResponse
+      )} directly.`
+    )
+  }
+}
+
 export async function wrapMiddlewares(
   middlewares: MiddlewareChain,
   routeFn: WinterSpecRouteFn<any, any, any>,
diff --git a/tests/errors/do-not-allow-raw-json.test.ts b/tests/errors/do-not-allow-raw-json.test.ts
index 8682796..62ae100 100644
--- a/tests/errors/do-not-allow-raw-json.test.ts
+++ b/tests/errors/do-not-allow-raw-json.test.ts
@@ -37,3 +37,31 @@ test("should throw an error when responding with raw JSON", async (t) => {
     )
   )
 })
+
+test("should throw a helpful error when route returns undefined", async (t) => {
+  const { axios } = await getTestRoute(t, {
+    globalSpec: {
+      authMiddleware: {},
+      beforeAuthMiddleware: [
+        async (req, ctx, next) => {
+          try {
+            return await next(req, ctx)
+          } catch (e: any) {
+            return Response.json({ error: e.message }, { status: 500 })
+          }
+        },
+      ],
+    },
+    routeSpec: {
+      methods: ["GET"],
+      jsonResponse: z.any(),
+    },
+    routePath: "/",
+    routeFn: (() => undefined) as any,
+  })
+
+  const { data } = await axios.get("/", {
+    validateStatus: () => true,
+  })
+  t.true(data.error.includes("returning undefined directly"))
+})