使用 Google 的计费 API 时，在 Google 的 API 中获取 NullPointerException

Question

这个问题与Google Marketplace App的计费API有关。我正在尝试按照以下位置提到的步骤获取客户许可证： http://code.google.com/googleapps/marketplace/v2/developers_guide.html#customer_license

我的代码是：

AppsMarketService service = new AppsMarketService();
service.appId = myAppID;
service.appName = "MyApp";
service.endpoint = "https://www.googleapis.com/appsmarket/v2/";
service.consumerKey = service.appId + ".apps.googleusercontent.com";
service.consumerSecret = MyAppSecretKey;
service.authorize();
CustomerLicense customerLicense = service.getCustomerLicense("example.com", true);

但出现以下异常：

java.lang.NullPointerException
    at com.google.appengine.api.urlfetch.URLFetchServiceImpl.fetch(URLFetchServiceImpl.java:31)
    at com.google.api.client.appengine.UrlFetchRequest.execute(UrlFetchRequest.java:80)
    at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:207)
    at com.google.api.client.sample.appsmarket.AppsMarketService.getCustomerLicense(AppsMarketService.java:117)
...

我正在使用以下 API：

appsmarket-2010_12_08.jar， 谷歌-api-client-1.2.1-alpha.jar， google-api-client-googleapis-1.2.1-alpha

请帮忙。