Access data in notifications

Hi, every one

I have problem when try to access data in notification, It's really strange with me

 @foreach(auth()->user()->unreadNotifications as $notification)                        
                            @foreach($notification->data as $data_item)   
                                {{dd($data_item)}}
                            @endforeach
                        @endforeach        

and here the result:

array:8 [▼
  "id" => 43
  "created_at" => "2018-03-11 22:59:35"
  "updated_at" => "2018-03-11 22:59:35"
  "subject" => "Sadgvasdfasd"
  "quote" => "
asdfasdfasdf
"
  "user_id" => 1
  "status" => 0
  "user" => array:7 [▶]
]

then when I write this:

{{($data_item['id'])}}

=> result is 43 but when I write this:

{{($data_item['subject'])}} 

=> I get this error: "Undefined index: subject.

thanks