sns/oauth2/access_token 接口返回null？

https://api.weixin.qq.com/sns/oauth2/access_token 接口返回null 这是什么原因，

INFO HttpMethodDirector:439 - I/O exception (java.net.ConnectException) caught when processing request: Connection refused

22:12:16,578  INFO HttpMethodDirector:445 - Retrying request

22:12:16,613  INFO HttpMethodDirector:439 - I/O exception (java.net.ConnectException) caught when processing request: Connection refused