sparse checkout bug?

I wanted to exclude a single massive directory from a checkout (the
LayoutTests directory from WebKit, which includes 130k files), so I
tried:

$ git config core.sparsecheckout true
$ echo '*' > .git/info/sparse-checkout
$ echo '!LayoutTests' >> .git/info/sparse-checkout
$ git read-tree -m -u HEAD

But LayoutTests is not being excluded. I also tried every variation of
including leading/trailing slash and trailing '*'.

Simple repro:

git init sparse-test && cd sparse-test &&
touch foo && mkdir bar && touch bar/baz &&
git add foo bar/baz &&
git commit -m initial &&
git config core.sparsecheckout true &&
echo '*' > .git/info/sparse-checkout &&
echo '!bar' >> .git/info/sparse-checkout &&
git read-tree -m -u HEAD &&
ls -1R
foo

bar:
baz

j.