[Rust] [Flexbuffers] HashMap with `Option<&str>` keys returns `KeyMustBeString` error even if every key in the map is `Some`

[Scripter17]

fn main() {
    flexbuffers::to_vec::<std::collections::HashMap<Option<&str>, &str, std::collections::hash_map::RandomState>>(std::collections::HashMap::from_iter(vec![(Some("a"), "a")])).unwrap();
}

serde_json only returns an error when there's a None key.