Unwrap on a None value

[moridius]

I have this very simple code:

fn main() {
    let rule = "FREQ=MONTHLY;BYDAY=1WE;INTERVAL=1;DTSTART=2018-06-05T18:00:00Z";
    Recurrence::from_str(&rule);
}

It crashes on me:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/rrules-0.2.3/src/recurrences/validations.rs:223:84

Am I missing something? :)