diff --git a/src/create-with-winter-spec.ts b/src/create-with-winter-spec.ts
index 229a28c..9e9e0e7 100644
--- a/src/create-with-winter-spec.ts
+++ b/src/create-with-winter-spec.ts
@@ -18,6 +18,7 @@ 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 { assertValidRouteReturnValue } from "./middleware/route-return-value-error.js"
 
 const attachMetadataToRouteFn = <
   const GS extends GlobalSpec,
@@ -126,6 +127,7 @@ function serializeResponse(
 ): Middleware {
   return async (req, ctx, next) => {
     const rawResponse = await next(req, ctx)
+    assertValidRouteReturnValue(rawResponse)
 
     const statusCode =
       rawResponse instanceof WinterSpecResponse
diff --git a/src/middleware/route-return-value-error.ts b/src/middleware/route-return-value-error.ts
new file mode 100644
index 0000000..ed3a950
--- /dev/null
+++ b/src/middleware/route-return-value-error.ts
@@ -0,0 +1,17 @@
+export const INVALID_ROUTE_RETURN_VALUE_MESSAGE =
+  "Use ctx.json({...}) instead of returning an object directly."
+
+export function isValidRouteReturnValue(value: unknown): boolean {
+  return (
+    value instanceof Response ||
+    (typeof value === "object" &&
+      value !== null &&
+      "serializeToResponse" in value)
+  )
+}
+
+export function assertValidRouteReturnValue(value: unknown) {
+  if (!isValidRouteReturnValue(value)) {
+    throw new Error(INVALID_ROUTE_RETURN_VALUE_MESSAGE)
+  }
+}
diff --git a/src/middleware/with-response-object-check.ts b/src/middleware/with-response-object-check.ts
index 9f8cae0..91530fe 100644
--- a/src/middleware/with-response-object-check.ts
+++ b/src/middleware/with-response-object-check.ts
@@ -1,6 +1,7 @@
 import { ResponseValidationError } from "./http-exceptions.js"
 import { Middleware } from "./types.js"
 import { RouteSpec } from "src/types/route-spec.js"
+import { INVALID_ROUTE_RETURN_VALUE_MESSAGE } from "./route-return-value-error.js"
 
 export const withResponseObjectCheck: Middleware<
   { routeSpec: RouteSpec<any> },
@@ -9,9 +10,7 @@ export const withResponseObjectCheck: Middleware<
   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."
-    )
+    throw new Error(INVALID_ROUTE_RETURN_VALUE_MESSAGE)
   }
 
   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..b63838d 100644
--- a/tests/errors/do-not-allow-raw-json.test.ts
+++ b/tests/errors/do-not-allow-raw-json.test.ts
@@ -37,3 +37,49 @@ test("should throw an error when responding with raw JSON", async (t) => {
     )
   )
 })
+
+const invalidReturnCases = [
+  { name: "undefined", value: undefined },
+  { name: "null", value: null },
+  { name: "string", value: "not a response" },
+  { name: "number", value: 42 },
+  { name: "boolean", value: true },
+  { name: "bigint", value: BigInt(42) },
+  { name: "symbol", value: Symbol("not-a-response") },
+  { name: "function", value: () => "not a response" },
+]
+
+for (const { name, value } of invalidReturnCases) {
+  test(`should throw ctx.json guidance when route returns ${name}`, 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"],
+        jsonBody: z.any(),
+        jsonResponse: z.any(),
+      },
+      routePath: "/",
+      routeFn: () => value as any,
+    })
+
+    const { data } = await axios.get("/", {
+      validateStatus: () => true,
+    })
+    t.true(
+      data.error.includes(
+        "Use ctx.json({...}) instead of returning an object directly"
+      )
+    )
+  })
+}