MessageSubscriberAsyncExtensions.FirstAsync is not working.

[c3-hoge-fuga-piyo]

Unity 6000.0.45f1

using UnityEngine;

using ZeroMessenger;

struct Message
{
}

sealed class Test : MonoBehaviour
{
    async void Start()
    {
        var broker = MessageBroker<Message>.Default;

        await broker.FirstAsync(this.destroyCancellationToken);
    }
}

ArgumentException: Message handler is already assigned.
ZeroMessenger.MessageBroker`1[T].Subscribe (ZeroMessenger.MessageHandler`1[T] handler) (at <35f312125e30466b9536671d1da5e9f6>:0)
ZeroMessenger.MessageSubscriberAsyncExtensions.FirstAsync[T] (ZeroMessenger.IMessageSubscriber`1[T] subscriber, System.Threading.CancellationToken cancellationToken) (at <35f312125e30466b9536671d1da5e9f6>:0)
Test.Start () (at Assets/Scripts/Test.cs:15)
System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state) (at <27bd554a9f0e46179afd19da1336e638>:0)
UnityEngine.UnitySynchronizationContext+WorkRequest.Invoke () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/UnitySynchronizationContext.cs:156)
UnityEngine.UnitySynchronizationContext.Exec () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/UnitySynchronizationContext.cs:84)
UnityEngine.UnitySynchronizationContext.ExecuteTasks () (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/UnitySynchronizationContext.cs:110)

[c3-hoge-fuga-piyo]

Workaround for this issue

subscriber.ToObservable().FirstAsync();