Testing for validation failures

Hi - I'm testing to ensure my validations are working as expected. The controller uses a FormRequest for validation. The test function is:

public function test_user_profile_validation_fails_when_first_is_missing()
    {
        $this->withoutExceptionHandling();

        $user = User::factory()->create();
        auth()->loginUsingId($user->id);

        $response = $this->post('profile/update/'.$user->id,
            [
                'first' => '',
                'last' => 'Hunt',
            ]);

        $response->assertStatus(302)
                 ->assertValidationErrors(['first']);
    }

and this is controller's update function:

public function update(ProfileRequest $request, User $user)
    {
        return route('profile',['user' => auth()->user()]);
    }

and the ProfileRequest

public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array<string, mixed>
     */
    public function rules()
    {
        return [
            'first' => ['string','required'],
            'last' => ['string','required'],
        ];
    }

I expect this test to pass, but I get an error AND the test fails:

 Illuminate\Validation\ValidationException : The first must be a string. (and 1 more error)

which is true but frustrates my ability to test the condition. I'm sure I've got something wrong and would appreciate your guidance! Thanks.