diff --git a/packages/bun-types/test.d.ts b/packages/bun-types/test.d.ts
index a3f7f9f8fe0474..1e96327290f068 100644
--- a/packages/bun-types/test.d.ts
+++ b/packages/bun-types/test.d.ts
@@ -146,7 +146,7 @@ declare module "bun:test" {
   export function spyOn<T extends object, K extends keyof T>(
     obj: T,
     methodOrPropertyValue: K,
-  ): Mock<T[K] extends (...args: any[]) => any ? T[K] : never>;
+  ): Mock<Extract<T[K], (...args: any[]) => any>>;
 
   interface FunctionLike {
     readonly name: string;
diff --git a/test/integration/bun-types/fixture/test.ts b/test/integration/bun-types/fixture/test.ts
index 968f07f960e087..eb4f710b65488e 100644
--- a/test/integration/bun-types/fixture/test.ts
+++ b/test/integration/bun-types/fixture/test.ts
@@ -1,9 +1,6 @@
-import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, spyOn, test } from "bun:test";
+import { afterAll, afterEach, beforeAll, beforeEach, describe, expect, type Mock, spyOn, test } from "bun:test";
 import { expectType } from "./utilities";
 
-const spy = spyOn(console, "log");
-expectType<any[][]>(spy.mock.calls);
-
 const hooks = [beforeAll, beforeEach, afterAll, afterEach];
 
 for (const hook of hooks) {
@@ -123,3 +120,24 @@ describe.each(dataAsConst)("test.each", (a, b, c) => {
   expectType<boolean>(b);
   expectType<5 | "asdf">(c);
 });
+
+const mySpyOnObjectWithOptionalMethod: {
+  optionalMethod?: (input: { question: string }) => { answer: string };
+} = {
+  optionalMethod: input => ({ answer: `Aswer to ${input.question}` }),
+};
+
+const mySpiedMethodOfOptional = spyOn(mySpyOnObjectWithOptionalMethod, "optionalMethod");
+mySpiedMethodOfOptional({ question: "asdf" });
+expectType<Mock<(input: { question: string }) => { answer: string }>>(mySpiedMethodOfOptional);
+
+const myNormalSpyOnObject = {
+  normalMethod: (name: string) => `Hello ${name}`,
+};
+
+const myNormalSpiedMethod = spyOn(myNormalSpyOnObject, "normalMethod");
+myNormalSpiedMethod("asdf");
+expectType<Mock<(name: string) => string>>(myNormalSpiedMethod);
+
+const spy = spyOn(console, "log");
+expectType(spy.mock.calls).is<[message?: any, ...optionalParams: any[]][]>();
diff --git a/test/regression/issue/18547.test.ts b/test/regression/issue/18547.test.ts
index d489177ccd78ad..31980159d00160 100644
--- a/test/regression/issue/18547.test.ts
+++ b/test/regression/issue/18547.test.ts
@@ -16,7 +16,6 @@ test("18547", async () => {
         expect(request.cookies.get("sessionToken")).toEqual("123456");
         expect(clone.cookies.get("sessionToken")).toEqual("654321");
 
-
         return new Response("OK");
       },
     },