The response is not a view message when unit test fails in laravel 5.5

I am new in laravel 5.5 unit testing.

I wrote a method in NewsController names index. It returns a view like below:

public function index(Request $request)
{
     $news = DB::table('news')->orderBy('created_at', 'desc')->get();
     $data['news'] = $news;
     return view('news.index')->with($data);
}

I wrote a test for that like below:

 public function testIndex(){
     factory(News::class, 10)->create();
    $news = DB::table('news')->orderBy('created_at', 'desc')->get();
    $this->get('/admin/news')
       ->assertViewHas('news', $news)
       ->assertStatus(200);
}

But test failed. The message is:

The response is not a view.

Can anyone help me?