Fails to serialize a struct with HashMap

The following is bugging the serializer:

    #[test]
    fn struct_with_hashmap_of_value() {
        use crate as zvariant;
        use serde::Serialize;

        let mut hm = HashMap::new();
        hm.insert("key".into(), Value::from(55));

        #[derive(Type, Serialize)]
        struct Foo {
            hmap: HashMap<String, Value<'static>>,
        }

        let foo = Foo { hmap: hm };

        let ctxt = Context::<LE>::new_dbus(0);
        to_bytes(ctxt, &foo).unwrap();
    }

thread 'tests::struct_with_hashmap_of_value' panicked at 'called Result::unwrap()on anErrvalue: Message("invalid type: charactera, expected s, g, oorv")', zvariant/src/lib.rs:738:9

When the struct has another field inside, the error is bit different. That might be another related issue.