Can't access object property in Laravel

I started learning Laravel since yesterday and face some difficulties to get property of an object.

Here is the class (created with artisan) :

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Task extends Model
{
    public $description;
    public $completed;
    public $timestamps = false;

    public function isComplete()
    {
        return $this->completed;
    }
}

And here is the issue :

>>> $tasks = App\Task::all();
=> Illuminate\Database\Eloquent\Collection {#654
     all: [
       App\Task {#652
         id: "1",
         description: "Learn Laravel",
         completed: "0",
       },
     ],
   }
>>> $tasks[0]->id
=> 1
>>> $tasks[0]->description
=> null

Why can I access the id but not the description ?

via JazZ