__Rewire__ is not a function

Question

Using Rewire for testing..

Rewire is failing to rewire, and I'm wondering why.

Here is an example of rewire failing:

A file has a few exported functions...

export function foo() {}
export function bar() {}
export function baz() {}

In a test file...

import * as pathFileHelpers from 'path/to/file';
pathFileHelpers.__Rewire__({
 readFileSync: () => {
 return 'test test test'
 },
})

_get__(...).__Rewire__ is not a function

Why?