How to return values from mocked method using test::spec

Is it possible to return fake values from mocked methods using an object created with `test::spec` ? I've tried this and it never seems to call my fake method:

```
$fakeValue = '1';
$mock = test::spec('FakeClass', [
'fakeMethod' => function () {
return $fakeValue;
}
])->construct();
```
Then when I call $mock->fakeMethod() it's not returning my fakeValue;

Is this just supported or am I doing something wrong?