Validation Error (Array to string conversion) in laravel 5.1

Hi guys i am struggling a couple of hours to fix this error but no luck please i need help about this error always says: (Array to string conversion)

Codes:

<?php

namespace App\Http\Controllers;

use Validator;
use Illuminate\Http\Request;
use App\Http\Controllers\Controller;

class PostController extends Controller
{
    public function postRegister(Request $request)
    {
        $validator = Validator::make($request->all(), [
         'firstname' => 'required',
         'lastname'  => 'required',
         'phone'     => 'required',
         'countries' => 'required',
         'city'      => 'required',
         'state'     => 'required',
         'address'   => 'required',
        ]);

        if ($validator->fails()) { // <-- i got an error this part i dont know why
            return redirect('register')
                        ->withErrors($validator)
                        ->withInput();
        }


    }
}

via Chebli Mohamed