Laravel Admin - Voyager route redirect

I have done laravel auth with is_admin field like:

if(auth()->user()->is_admin == 1){
        return $next($request);
    }

and defined it in route so if the user is admin, can be redirected to voyager dashboard page.

Route::group(['prefix' => 'admin'], function()
{
    Voyager::routes();
    Route::get('admin', 'HomeController@admin')->middleware('admin');

});

When I login, it's redirected to specific page but with error message:

Target class [VoyagerController] does not exist.

What should I do to return voyager admin page?

via Chebli Mohamed