git subtree add fails in new repository

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.