尝试获取长期访问令牌时出现未知错误

我正在尝试使用以下代码为我的一个页面获取长期访问令牌：

$page_info = $facebook->api("/page-id?fields=access_token");
$args = array(
    "client_id" => $facebook->getAppId(),
    "client_secret" => $facebook->getAppSecret(),
    "grant_type" => "fb_exchange_token",
    "fb_exchange_token" => $page_info["access_token"]
);
$result = $facebook->api("/oauth/access_token", $args);
echo json_encode($result);

。但我得到这个响应：{"error_code"：1，"error_msg"："发生未知错误"}

如果我浏览到

https://graph.facebook.com/oauth/access_token?             
    client_id=APP_ID&
    client_secret=APP_SECRET&
    grant_type=fb_exchange_token&
    fb_exchange_token=EXISTING_ACCESS_TOKEN 

如 https://developers.facebook.com/roadmap/offline-access-removal/#page_access_token 所述