PHP 公众号的高级群发接口不能上传图文消息素材，返回errcode:-2 errmsg:''-PHP中文网问答

POST数据到https://api.weixin.qq.com/cgi-bin/media/uploadnews?access_token=ACCESS...（ACCESS_TOKEN是我自己的），返回如下：
array
'errcode' => int -2
'errmsg' => string '' (length=0)

请问这是什么问题？

--------------------------- 原代码 ----------------------------
$news[] = array("thumb_media_id"=>$image_list[0], "author"=>"kxc", "title"=>"最新公告1", "content_source_url"=>"lejia51.com", "content"=>"--", "digest"=>"notice", "show_cover_pic"=>"1");

$news[] = array("thumb_media_id"=>$image_list[1], "author"=>"kxc", "title"=>"最新公告2", "content_source_url"=>"lejia51.com", "content"=>"--", "digest"=>"notice", "show_cover_pic"=>"0");

$url = "https://api.weixin.qq.com/cgi-bin/media/uploadnews?access_token=" . $this->token();

$json = json_encode(array("articles"=>$news));

$curl = curl_init();
curl_setopt($curl, CURLOPT_URL, $url);
curl_setopt($curl, CURLOPT_HEADER, FALSE);
curl_setopt($curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($curl, CURLOPT_POSTFIELDS, $json);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, FALSE);
curl_setopt($curl, CURLOPT_USERAGENT, "Mozilla/5.0 (compatible; MSIE 5.01; Windows NT 5.0)");
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($curl, CURLOPT_AUTOREFERER, 1);

$data = curl_exec($curl);

if (curl_errno($curl)) {
$this->write_file(curl_error($curl));
return null;
}

$result = json_decode($data, true);

curl_close($this->cur);

var_dump($result);