$ git submodule add /path/to/dir/with/another/repo test
fatal: You are on a branch yet to be born
Unable to checkout submodule 'test'

Next time I called same

$ git submodule add /path/to/dir/with/another/repo test
Adding existing repo at 'test' to the index
warning: LF will be replaced by CRLF in .gitmodules.
The file will have its original line endings in your working directory.