JSON encoded list Type for POST call?

I am trying to do a POST call through an API but it doesn't really work.

When removing the items however it works fine.. The API requires items to be a JSON Encoded list.. What am I doing wrong?

$items = array(
    "description" => $_GET['description'],
    "sub_type" => $subtype_fd,
    "amount" => floatval($_GET['amount']),
    "calcium" => intval($_GET['calcium']),
    "calories" => intval($_GET['calories']),
    "carbohydrate" => floatval($_GET['carbohydrate']),
    "cholesterol" => floatval($_GET['cholesterol']),
    "fiber" => floatval($_GET['fiber']),
    "protein" => floatval($_GET['protein']),
    "saturated_fat" => floatval($_GET['saturated_fat']),
    "unsaturated_fat" => floatval($_GET['unsaturated_fat']),
    "sodium" => floatval($_GET['sodium']),
    "sugar" => floatval($_GET['sugar']),
    "caffeine" => floatval($_GET['caffeine'])
);

$json = json_encode($items);
var_dump($json);
$data = array('note' => $_GET['title'], 'sub_type' => $subtype_bld, 'items' => $json);