Laravel 5 Model is not updating with null values

I have this Invoice model where I try to update member_id and district_id with null:

#attributes: array:14 [▼
    "id" => 106
    "number" => "20150505"
    "date" => "2015-09-22 20:29:48"
    "member_id" => null
    "district_id" => null
    "description" => " "
    "items" => " "
    "notes" => "uolo"
    "po_number" => "123"
    "amount" => "0"
    "status" => "Paid"
    "created_at" => "2015-09-22 19:20:32"
    "updated_at" => "2015-09-22 20:26:41"
    "receiver_details" => """
      yono\r\n
      asd\r\n
      ads\r\n
      \r\n
      sad
      """
]
#original: array:14 [▼
    "id" => 106
    "number" => 20150505
    "date" => "2015-09-22 20:26:41"
    "member_id" => 166
    "district_id" => 13
    "description" => " "
    "items" => " "
    "notes" => "uolo"
    "po_number" => "123"
    "amount" => 0.0
    "status" => "Paid"
    "created_at" => "2015-09-22 19:20:32"
    "updated_at" => "2015-09-22 20:26:41"
    "receiver_details" => "yono"

]

The update is like this:

$invoice = $this->invoice->find($invoice_id);
$invoice->number = $number;
$invoice->date = Carbon::createFromFormat('m/d/Y', $date);
$invoice->member_id = $member_id;
$invoice->district_id = $district_id;
$invoice->description = $description;
$invoice->notes = $notes;
$invoice->items = $items;
$invoice->po_number = $po_number;
$invoice->amount = $amount;
$invoice->status = $status;
$invoice->receiver_details = $receiver_details;
dd($invoice);
$invoice->save();

What am I missing? The problem is that the member_id remains the original one and yes, the field in nullable.

via Chebli Mohamed