Laravel 5.3 Call to undefined method Illuminate\Database\Eloquent\Factory::state()

I'm trying to define a few different variations of a User model for testing, using Laravels ModelFactory as documented here

$factory->define(App\User::class, function(\Faker\Generator $faker) {
    return [
        'name' => $faker->name,
        'email' => $faker->unique()->safeEmail,
        'remember_token' => str_random(10),
        'phone' => $faker->phoneNumber,
    ];
});

$factory->state(App\User::class, 'admin', function (Faker\Generator $faker) {
    return [
        'groups' => function(App\User $u) {
            return App\Models\Group::where('level', '<=', 5)->get()->toArray();
        }
    ];
});

And then I create a User model:

$user = factory(User::class)->states('admin')->make();

But phpunit seems to exit out of the test without complaining. In the PHP logs, I see:

Call to undefined method Illuminate\Database\Eloquent\Factory::state()

There isn't very much documentation on the state() method in Laravel docs, and I've searched and experimented for hours with no progress to show for it.

As a sidenote: the groups attribute is referring to a Many relationship. However, this Exception is thrown regardless of which model I am creating, even simple models.

via Chebli Mohamed