diff --git a/src/create-with-winter-spec.ts b/src/create-with-winter-spec.ts
index 229a28c..79e4303 100644
--- a/src/create-with-winter-spec.ts
+++ b/src/create-with-winter-spec.ts
@@ -17,7 +17,10 @@ import { withMethods } from "./middleware/with-methods.js"
 import { withInputValidation } from "./middleware/with-input-validation.js"
 import { withUnhandledExceptionHandling } from "./middleware/with-unhandled-exception-handling.js"
 import { ResponseValidationError } from "./middleware/http-exceptions.js"
-import { withResponseObjectCheck } from "./middleware/with-response-object-check.js"
+import {
+  assertRouteReturnsResponse,
+  withResponseObjectCheck,
+} from "./middleware/with-response-object-check.js"
 
 const attachMetadataToRouteFn = <
   const GS extends GlobalSpec,
@@ -126,6 +129,7 @@ function serializeResponse(
 ): Middleware {
   return async (req, ctx, next) => {
     const rawResponse = await next(req, ctx)
+    assertRouteReturnsResponse(rawResponse)
 
     const statusCode =
       rawResponse instanceof WinterSpecResponse
diff --git a/src/middleware/with-response-object-check.ts b/src/middleware/with-response-object-check.ts
index 9f8cae0..99461d0 100644
--- a/src/middleware/with-response-object-check.ts
+++ b/src/middleware/with-response-object-check.ts
@@ -1,18 +1,37 @@
-import { ResponseValidationError } from "./http-exceptions.js"
 import { Middleware } from "./types.js"
 import { RouteSpec } from "src/types/route-spec.js"
 
+export const RAW_RESPONSE_OBJECT_ERROR_MESSAGE =
+  "Use ctx.json(value) or another Response object instead of returning raw route values directly."
+
+function hasWinterSpecSerializer(
+  rawResponse: unknown
+): rawResponse is { serializeToResponse: (...args: any[]) => Response } {
+  return (
+    typeof rawResponse === "object" &&
+    rawResponse !== null &&
+    "serializeToResponse" in rawResponse &&
+    typeof rawResponse.serializeToResponse === "function"
+  )
+}
+
+export function assertRouteReturnsResponse(rawResponse: unknown) {
+  if (rawResponse instanceof Response || hasWinterSpecSerializer(rawResponse)) {
+    return
+  }
+
+  throw new Error(
+    `${RAW_RESPONSE_OBJECT_ERROR_MESSAGE} Route handlers must return a Response or ctx.json(...).`
+  )
+}
+
 export const withResponseObjectCheck: Middleware<
   { routeSpec: RouteSpec<any> },
   {}
 > = async (req, ctx, next) => {
   const rawResponse = await next(req, ctx)
 
-  if (typeof rawResponse === "object" && !(rawResponse instanceof Response)) {
-    throw new Error(
-      "Use ctx.json({...}) instead of returning an object directly."
-    )
-  }
+  assertRouteReturnsResponse(rawResponse)
 
   return rawResponse
 }
diff --git a/tests/errors/do-not-allow-raw-json.test.ts b/tests/errors/do-not-allow-raw-json.test.ts
index 8682796..f2e8780 100644
--- a/tests/errors/do-not-allow-raw-json.test.ts
+++ b/tests/errors/do-not-allow-raw-json.test.ts
@@ -1,8 +1,16 @@
-import test from "ava"
+import test, { type ExecutionContext } from "ava"
 import { z } from "zod"
 import { getTestRoute } from "tests/fixtures/get-test-route.js"
+import { RAW_RESPONSE_OBJECT_ERROR_MESSAGE } from "src/middleware/with-response-object-check.js"
+import type {
+  SerializableToResponse,
+  WinterSpecRouteFn,
+} from "src/types/web-handler.js"
 
-test("should throw an error when responding with raw JSON", async (t) => {
+const getInvalidResponseError = async (
+  t: ExecutionContext,
+  routeFn: WinterSpecRouteFn<any, any, any>
+) => {
   const { axios } = await getTestRoute(t, {
     globalSpec: {
       authMiddleware: {},
@@ -23,17 +31,92 @@ test("should throw an error when responding with raw JSON", async (t) => {
       jsonResponse: z.any(),
     },
     routePath: "/",
-    routeFn: (req, ctx) => {
-      return { foo: "bar" } as any
-    },
+    routeFn,
   })
 
   const { data } = await axios.get("/", {
     validateStatus: () => true,
   })
-  t.true(
-    data.error.includes(
-      "Use ctx.json({...}) instead of returning an object directly"
-    )
-  )
+
+  return data.error
+}
+
+const assertInvalidRawReturnError = (t: ExecutionContext, error: string) => {
+  t.true(error.includes(RAW_RESPONSE_OBJECT_ERROR_MESSAGE))
+}
+
+test("should throw an error when responding with raw JSON", async (t) => {
+  const error = await getInvalidResponseError(t, () => {
+    return { foo: "bar" } as any
+  })
+
+  assertInvalidRawReturnError(t, error)
+})
+
+for (const [label, value] of [
+  ["string", "plain text"],
+  ["number", 200],
+  ["boolean", true],
+  ["bigint", 200n],
+  ["symbol", Symbol("raw")],
+  ["function", () => "raw"],
+] as const) {
+  test(`should throw an error when responding with raw ${label}`, async (t) => {
+    const error = await getInvalidResponseError(t, () => {
+      return value as any
+    })
+
+    assertInvalidRawReturnError(t, error)
+  })
+}
+
+test("should throw an error when responding with null", async (t) => {
+  const error = await getInvalidResponseError(t, () => {
+    return null as any
+  })
+
+  assertInvalidRawReturnError(t, error)
+})
+
+test("should throw an error when responding with undefined", async (t) => {
+  const error = await getInvalidResponseError(t, () => {
+    return undefined as any
+  })
+
+  assertInvalidRawReturnError(t, error)
+})
+
+test("should allow custom serializable response objects", async (t) => {
+  class PlainTextResponse implements SerializableToResponse {
+    statusCode() {
+      return 200
+    }
+
+    serializeToResponse() {
+      return new Response("custom response", {
+        headers: { "content-type": "text/plain" },
+      })
+    }
+  }
+
+  const { axios } = await getTestRoute(t, {
+    globalSpec: {
+      authMiddleware: {},
+    },
+    routeSpec: {
+      methods: ["GET"],
+      jsonResponse: z.object({
+        message: z.string(),
+      }),
+    },
+    routePath: "/",
+    routeFn: () => new PlainTextResponse() as any,
+  })
+
+  const response = await axios.get("/", {
+    responseType: "text",
+  })
+
+  t.is(response.status, 200)
+  t.is(response.data, "custom response")
 })