How do i get Android JSON Object data on Laravel 5.4

my web php; Route::get('/checkUser/{user?}', 'CheckUserController@index');

my controller; class CheckUserController extends Controller { public function index($user){

$data=$user;

return response()->json([
    'contacts' => $data
]);

} }

But android error: Value <! DOCTYPE of type java.lang.String cannot be converted to JSONObject