Get acting as user's id when testing in Laravel

Laravel 5.7. I have a test like this:

/** @test */
public function do_nothing_test()
{
    $user = factory(User::class)->states('app')->make();
    $this->actingAs($user, 'api');
    $response = $this->postJson('some_endpoint');
}

which calls an endpoint in a controller that tries to get the logged in user:

public function useless_endpoint()
{
    $id = auth()->user()->id;
}

The value of $id is null. How can I get the test to work so that the user's details are available to the controller?