How to access total Amount Spent in Facebook API Java for Business accounts

Question

I am creating an analytics page for my website. So I need to get data from facebook API. My current code is

public class HelloFacebookAPI {

    public static final String ACCESS_TOKEN = [TOKEN];
    public static final long ACCOUNT_ID = [ACCOUNT_ID];
    public static final String APP_SECRET = [APP_SECRET];

    public static final APIContext context = new APIContext(ACCESS_TOKEN, APP_SECRET);
    public static void main(String[] args) {
        try {
            AdAccount account = new AdAccount(ACCOUNT_ID, context);
            System.out.println(account.getFieldAmountSpent());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

But this keeps returning null. My account is a Business account. Also and Ad account is associated with the account. How can I make this work?