ReflectionMethod of ArrayObject constructor - getNumberOfRequiredParameters bug?

We can create a new instances of ArrayObject passing an array as constructor parameter

$arrayObject = new ArrayObject([1,2,3]);

but that is also fine:

$arrayObject = new ArrayObject();

However

$arrayObjectReflection = new ReflectionClass('ArrayObject');
$arrayObjectReflection->getConstructor()->getNumberOfRequiredParameters();

Returns 1.

Am I misunderstand optional parameters in PHP or that is just a bug?

P.S: I'm using PHP 5.5.9 (Zend Engine)