XML mapping doesn't map XmlKeyValuePairs but works for JSON

[bentcoder]

Does anyone know why XML mapping for extended property doesn't work as it does for JSON version?

MODEL

namespace My\Bundle\Model;

use JMS\Serializer\Annotation as Serializer;

/**
 * @Serializer\XmlRoot("product")
 */
class Create
{
    /**
     * @Serializer\Type("string")
     */
    public $title;

    /**
     * @Serializer\Type("array<string, string>")
     * @Serializer\XmlKeyValuePairs
     */
    public $extended;
}

JSON REQUEST

{
    "title": "Created name",
    "extended": {
        "crt_key_1": "crt_val_1",
        "crt_key_2": "crt_val_2"
    }
}

Mapping result

My\Bundle\Model\Create Object
(
    [title] => Created name
    [extended] => Array
        (
            [crt_key_1] => crt_val_1
            [crt_key_2] => crt_val_2
        )
)

XML REQUEST

<?xml version="1.0" encoding="UTF-8"?>
<product>
    <title>Created name</title>
    <extended>
        <crt_key_1>crt_val_1</crt_key_1>
        <crt_key_2>crt_val_2</crt_key_2>
    </extended>
</product>

Mapping result

My\Bundle\Model\Create Object
(
    [title] => Created name
    [extended] => Array
        (
        )
)