PEBKAC or bug: unable to create path-like branch names

I want to create a nested feature branch, but git keeps complaining if I nest more than one level deep:

    $ git checkout -b dev/feature/foo
    error: unable to resolve reference refs/heads/dev/feature/foo:
    Not a directory
    fatal: Failed to lock ref for update: Not a directory

Based on my reading of the manual pages, it seems like I should be able to nest branch names as long as it conforms to certain rules. I read git-branch(1), which points me to git-check-ref-format(1), which seems to say that the rules are being followed.

On the other hand, running:

    $ git check-ref-format foo; echo $?

always results in a non-zero error code, even with a literal 'foo' as a branch name, so clearly it isn't saying what I think it's saying. *shrug*

Can someone provide a little clarity here?