REST API "Update Folder" does not work fine

Hi all

I tried to call "Update Folder" and it returns success, but the folder is changed nothing.
API doc.
http://developers.marketo.com/documentation/asset-api/update-folder/

My Request JSON (Change description text) 
{
"description": "new description"
}

My Response JSON
{"success":true,"warnings":[],"errors":[],"requestId":"25a5#14c6e85a82d","result":[{"id":39,"name":"Z_正木","description":null,"createdAt":"2014-10-01T08:08:00Z+0000","updatedAt":"2014-10-01T08:08:00Z+0000","type":"Marketing Folder","parent":null,"path":"/Marketing Activities/Default/Z_正木","isArchive":false,"isSystem":false,"workspace":"Default"}]}