gapi.client.plus.people.get 返回 403 错误

我正在为一个网页做一个appengine项目。我在开发者控制台中打开了 Google+ API。网页登录方式为：

userService.createLoginURL(request.getRequestURI())

之后，我想检索已登录用户的个人资料图片。为此，我导入了 js 库：

<script src = "https://plus.google.com/js/client:platform.js" async defer></script>

和：

<script src="https://apis.google.com/js/client:plusone.js" type="text/javascript"></script>

然后我执行：

gapi.client.load('plus','v1', function(){
    var request = gapi.client.plus.people.get({
        'userId': 'me'
    });
    request.execute(function(resp) {
        console.log('Image at:' + resp.image.url);
    });
});

但这会继续返回 403 错误：

{
 "error": {
  "errors": [
   {
    "domain": "usageLimits",
    "reason": "dailyLimitExceededUnreg",
    "message": "Daily Limit for Unauthenticated Use Exceeded. Continued use requires signup.",
    "extendedHelp": "https://code.google.com/apis/console"
   }
  ],
  "code": 403,
  "message": "Daily Limit for Unauthenticated Use Exceeded. Continued use requires signup."
 }
}

我该如何解决这个问题？我错过了什么？