Unserialized reflection class cause error.

<?php
/**
 * abc
 */
class a{}

$ref = new ReflectionClass('a');
$ref = unserialize(serialize($ref));
var_dump($ref);
var_dump($ref->getDocComment());

// object(ReflectionClass)#2 (1) {
//   ["name"]=>
//   string(1) "a"
// }
// PHP Fatal error:  ReflectionClass::getDocComment(): Internal error: Failed to retrieve the reflection object
?>