bash

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/work (master)
$ git submodule add file:///C/Users/Eugene/Desktop/tmp_git test
fatal: You are on a branch yet to be born
Unable to checkout submodule 'test'

---------------------------------------------------------------

Main repo:

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/work (master)
$ git branch -a
  experiment
* master

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/work (master)
$ git rev-parse HEAD
edad021cea1e47d32d1d9cd33d6db6fdf07db3a9

===============================================================

Main repo submodule:

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/work/test (master)
$ git branch -a

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/work/test (master)
$ git rev-parse HEAD
HEAD
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

===============================================================

Submodule origin:

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/tmp_git (master)
$ git branch -a
* master

Eugene@EUGENE-PC /C/Users/Eugene/Desktop/tmp_git (master)
$ git rev-parse HEAD
39e5fa0188982bee6aae8e4acd47840f362a173d