Sinon.js: Override or replace function in a module with fake/stub

Question

I want to just assert that f2 calls f1 without having to worry about the type of arguments and how they're used. How do I achieve it?

module.js

export function f1(arg1, arg2) {
  // do something with args
}
export function f2() {
  f1()
}

module.spec.ts

import * as moduleToBeTested from './module';

test('f2 calls f1', () => {
  // here I want to replace function f1 with a stub/fake and just assert that it's called but the following doesn't work
  const f1Stub = sinon.stub(moduleToBeTested, 'f1');
  moduleToBeTested.f2();
  sinon.assert.calledOnce(f1Stub);
})