从GAE API使用endpointscfg生成Java客户端库时出现HTTP 500错误

I tried to generate java client library using a discovery doc retrieved from my API endpoints (which is running on golang) but failed wit the following message.

endpointscfg.py gen_client_lib java happylaundry.rest.discovery

Traceback (most recent call last):
  File "//Users/lorenz/go/go_appengine/endpointscfg.py", line 133, in <module>
    run_file(__file__, globals())
  File "//Users/lorenz/go/go_appengine/endpointscfg.py", line 129, in run_file
    execfile(_PATHS.script_file(script_name), globals_)
  File "/Users/lorenz/go/go_appengine/google/appengine/tools/endpointscfg.py", line 561, in <module>
    main(sys.argv)
  File "/Users/lorenz/go/go_appengine/google/appengine/tools/endpointscfg.py", line 557, in main
    args.callback(args)
  File "/Users/lorenz/go/go_appengine/google/appengine/tools/endpointscfg.py", line 458, in _GenClientLibCallback
    args.build_system)
  File "/Users/lorenz/go/go_appengine/google/appengine/tools/endpointscfg.py", line 335, in _GenClientLib
    build_system, client_name)
  File "/Users/lorenz/go/go_appengine/google/appengine/tools/endpointscfg.py", line 366, in _GenClientLibFromContents
    raise ServerRequestException(error)
__main__.ServerRequestException: HTTP 500 (Internal Server Error) error when communicating with URL: https://google-api-client-libraries.appspot.com/generate. Response: 

I tried to use the same command and generated for a very simple program, and it runs without errro. From the syntax or structure of the discovery doc, i can't see anything ran wrong. Do you guys have any idea?