Testing Inertia

Who can help me out here?

I'm trying to test some (not Breeze or Fortify) authentication routes, while using Inertia.

The following (PEST) test tells me that that "Invalid JSON was returned from the route."

A quick look at Dev Tools in Chrome reveals the JSON is fine though. Any idea what's going wrong here?

it('returns a failed boolean if authentication fails', function () {
    $this->followingRedirects()
        ->post('/login', [
            'email' => '[email protected]',
            'password' => 'password'
        ])->assertJsonFragment(['failed' => true]);
});

I've had a look at Inertia Laravel Testing Helpers docs but it looks a bit crazy and over-the-top for me (at the moment).