新手编译失败，求指教！

Traceback (most recent call last):
  File "/home/huangkai/Toybrick/.repo/repo/main.py", line 549, in <module>
    _Main(sys.argv[1:])
  File "/home/huangkai/Toybrick/.repo/repo/main.py", line 524, in _Main
    result = repo._Run(argv) or 0
  File "/home/huangkai/Toybrick/.repo/repo/main.py", line 186, in _Run
    result = cmd.Execute(copts, cargs)
  File "/home/huangkai/Toybrick/.repo/repo/subcmds/sync.py", line 816, in Execute
    fetched = self._Fetch(to_fetch, opt)
  File "/home/huangkai/Toybrick/.repo/repo/subcmds/sync.py", line 416, in _Fetch
    self._GCProjects(projects)
  File "/home/huangkai/Toybrick/.repo/repo/subcmds/sync.py", line 437, in _GCProjects
    bare_git.gc('--auto')
  File "/home/huangkai/Toybrick/.repo/repo/project.py", line 2868, in runner
    (self._project.name, name, p.stderr))
error.GitError: platform/external/nanohttpd gc: fatal: not a git repository: '/home/huangkai/Toybrick/.repo/projects/external/nanohttpd.git'

我现在改成用Ubuntu系统安装编译，在repo sync过程中遇见这个问题，请问如何解决？