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？

多拉几次试试，有可能是网络问题。