Rust bug in Unicode handling?

console-dump.txt

00:25:30 erszcz @ x2 : /tmp
$ cat zoladz.rs 
fn main() {
    let z = "żołądź";
    println!("{:c}", z.char_at(1));
}
00:25:33 erszcz @ x2 : /tmp
$ rustc zoladz.rs 
00:25:37 erszcz @ x2 : /tmp
$ ./zoladz 
task '<main>' failed at 'assertion failed: (w != 0)', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libcore/str.rs:2059
00:25:38 erszcz @ x2 : /tmp
$ rustc --version
rustc 0.12.0-nightly (b5ba2f551 2014-10-06 20:27:14 +0000)

zoladz.rs

fn main() {
    let z = "żołądź";
    println!("{:c}", z.char_at(1));
}