Unable to use factory() in Test's setUp() method in Laravel - php

Question

I've encountered an error when refactoring my Laravel tests by adding at setUp() method.
This works as expected:
<?php
namespace Tests\Feature;
use App\Models\Person;
use Tests\TestCase;
class MembershipTicketsTest extends TestCase
{
public function test_a_non_member_cannot_list_redeemable_events()
{
$user = Person::factory()->create();
$this->actingAs($user);
$this->get(route('event.index'))->assertStatus(302)->assertRedirect(route('membership.create'));
}
Where as this returns the error Unknown formatter "firstName":
<?php
namespace Tests\Feature;
use App\Models\Person;
use Tests\TestCase;
class MembershipTicketsTest extends TestCase
{
protected $user;
protected function setUp() : void
{
$this->user = Person::factory()->create();
}
public function test_a_non_member_cannot_list_redeemable_events()
{
$this->actingAs($this->user);
$this->get(route('event.index'))->assertStatus(302)->assertRedirect(route('membership.create'));
}
Every solution to this error I've found says to make sure that you have use Tests\TestCase; in your file, which I do.
I presume this means I cannot use factories in the setUp() method of a test in Laravel. Is this true, and why is it the case?