vi.spyOn does not work

gistfile1.txt

==== example.js ====

function sub1() {
  return 'hello'
}
function main1() {
  return sub1()
}
export { sub1, main1 }

==== example.test.js ====

import { describe, expect, it, vi } from "vitest";
import * as api from './example'

describe('main', () => {
  it("example", () => {
    const spy1 = vi.spyOn(api, 'sub1')
    api.main1()
    expect(spy1).toHaveBeenCalledTimes(1)
  })
})