git subtree add fails in new repository

[Daurnimator <quae@xxxxxxxxxxxxxxx>]

I'm trying to make a series of repository that only contain subtrees
for various other projects.
However git subtree does not like being on a newly created branch:

$ git init
$ git subtree add --prefix=git https://github.com/git/git.git master
fatal: ambiguous argument 'HEAD': unknown revision or path not in the
working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Working tree has modifications.  Cannot add.