How to stub a dependency from the same module as its caller?

Really new to ES6 modules AND automated testing, and I’m stumped!

I’m using mocha/chai/sinon, and unit testing has been going reasonably well, but I’m having trouble figuring out how to isolate “doOtherStuff” in this pattern:

module file

export var doStuff = function doStuff(){ be statements...

export var doOtherStuff = function doOtherStuff(){
  const someValue = doStuff(); be more statements...

Test file

import * as Things from 'path/to/module'

describe('does it work?', function(){
  it('runs as I expect it to', function(){
    const result = sinon.stub(Things, 'doStuff');
    const theTest = Things.doOtherStuff();
    ...chai.expect code...

When the test runs, it errors out on a chunk of code in doStuff (Cannot read property ‘indexOf’ of undefined), which I thought I had stubbed. Is stubbing a dependency from the same module as its caller impossible because of the module’s namespacing or something? I suspect there’s a workaround, but anything I’ve turned up in my googling doesn’t seem to work or doesn’t seem to address this particular pattern. Any help is greatly appreciated!