After null elimination, data disappears, is it a bug or a feature?

[lav45]

$target = array(
  true, 
  array(
    "variable1" => 100000,
    "variable2" => "Hello, World!",
    "Hello!"
  ),
  0.234, 
  0, 
  null, 
  590834290589032580
);
$encoded_data = \CBOR\CBOREncoder::encode($target);
$decoded_variable = \CBOR\CBOREncoder::decode($encoded_data);
var_dump($decoded_variable);

Response:

array(4) {
  [0]=> bool(true)
  [1]=> array(3) {
    ["variable1"]=> int(100000)
    ["variable2"]=> string(13) "Hello, World!"
    [0]=> string(6) "Hello!"
  }
  [2]=> float(0.234)
  [3]=> int(0)
}