Git stash -u deleting files

# in a fresh repository
$ git --version
git version 2.17.1
$ mkdir foo && touch -- foo/bar
$ git stash push -u -m "test" foo
Saved working directory and index state On improvement/pam2-2779-add-types-to-entitiy-models: test
fatal: pathspec 'foo' did not match any files
error: unrecognized input
$ git stash show stash@{0}
# no output
$ ls -l
total 0