34 Laravel テーブル更新エラーfillable

LaravelではEloquentでfillでアップデートしようとした時、
fillable で更新カラムが許可されてないと、下記のエラーが出ます。

Add ［key］ to fillable property to allow mass assignment on

$ples = new Plesk;
$plesk = 'plesk_license';
$ret = $plesk->select()->where('id', $id)->first();
$ret->fill(['plesk_key' => $this->keyNumber, 'keyfile' => $this->key]);
$ret->timestamps = false;
$res = $ret->save();

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Plesk extends Model
{

  public $connection = '';
  public $table = '';
  public $fillable  = ['plesk_key','key_num','keyfile','key_file'];

}