Test error

a Resultado del test.txt

Testing started at 13:17 ...
sftp://root@127.0.0.1:2222/usr/bin/php /var/www/vendor/phpunit/phpunit/phpunit --configuration /var/www/phpunit.xml --filter "/::Usuario_puede_crear_clientes( .*)?$/" Tests\\Feature\\ClientTest /var/www/tests/Feature/ClientTest.php --teamcity
PHPUnit 7.0.2 by Sebastian Bergmann and contributors.


null does not match expected type "array".
 /var/www/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestResponse.php:709
 /var/www/tests/Feature/ClientTest.php:114
 


Time: 31.5 seconds, Memory: 24.00MB


FAILURES!
Tests: 1, Assertions: 18, Failures: 1.

Process finished with exit code 1

ClientTest.php

public function Usuario_puede_crear_clientes()
    {
        //Verifica que salta mensaje de error en caso de que los campos estén vacios
        $this->users=factory(User::class)->create();
        $this->withExceptionHandling();
        $this->signIn();
        $this->get('/clientes');
        $this->clients->nombre=null;
        $this->clients->apellidos=null;
        $this->clients->nif=null;
        $response=$this->post('/clientes/',$this->clients->toArray());
        $response
            ->assertStatus(302)
            ->assertSessionHasErrors(['nombre','apellidos','nif']);

        $this->clients->nombre='Ratoncito';
        $response=$this->post('/clientes',$this->clients->toArray());
        $response
            ->assertStatus(302)
            ->assertSessionHasErrors(['apellidos','nif']);

        $this->clients->apellidos='Perez';
        $response=$this->post('/clientes',$this->clients->toArray());
        $response
            ->assertStatus(302)
            ->assertSessionHasErrors(['nif']);

        //verifica que el nif no es válido
        $this->clients->nif='10002112P';
        $response=$this->post('/clientes',$this->clients->toArray());
        $response
            ->assertStatus(302)
            ->assertSessionHasErrors(['nif']);



        //Verifica que los campos obligatorios son correctos y válidos, y además devuelva el mensaje de cliente creado.
        $this->client = factory(Client::class)->create([
            'nombre' => 'Abigail',
            'apellidos'=> 'Pestaña Rota',
            'nif'=>'10002112X',
            'user'=>$this->users->id
        ]);


        $response=$this->post('/clientes/',$this->clients->toArray());


        $response
            ->assertStatus(302)
            ->assertSee('clientes')
            ->assertSessionHas('message',['success',__('Cliente creado correctamente')]); //esta es la linea que falla