Mysterious runtime failures when chaining methods on a Timer

In timer.rs:

``` rust
use std::io::timer::Timer;

fn main(){
    let wait: Receiver<()> = Timer::new().unwrap().oneshot(5000);
    wait.recv();
}
```

Result:

```
task '<main>' failed at 'Box<Any>', /path/to/rust/src/libstd/comm/mod.rs:763
```

Unchaining the `oneshot` method call results in normal behaviour.
