Subjects and join patterns with Visual Rx

Question

I am using the excellent Visual Rx library in f# but in combination with join patterns or subjects i get a: NullReferenceException 

   at System.Reactive.Contrib.Monitoring.MonitorOperator`1.<.ctor>b__0(T item, MarbleBase marble) in c:\Code\Rx\VisualRx\Monitor\Code Side\System.Reactive.Contrib.Monitoring\MonitorOperator.cs:line 68
   at System.Reactive.Contrib.Monitoring.Contracts.MarbleNext.Create[T](String name, MarbleSerializationOptions options, T item, Func`3 valueStrategy, TimeSpan elapsed, String machineName) in c:\Code\Rx\VisualRx\Monitor\System.Reactive.Contrib.Monitoring.Contracts\[Marble]\MarbleNext.cs:line 48
   at System.Reactive.Contrib.Monitoring.MonitorOperator`1.PublishOnNext(T value) in c:\Code\Rx\VisualRx\Monitor\Code Side\System.Reactive.Contrib.Monitoring\MonitorOperator.cs:line 172
   at System.Reactive.Linq.Observαble.Do`1._.OnNext(TSource value)

        let subj = new BehaviorSubject<unit> ()
        let disp = subj.Monitor("BehaviourSubject1",1.).Subscribe()

 

I also get a NullReference when monitoring an observable that i created through join patterns where the containing observables can be monitored without problems. I do not see an obvious correspondence to the temperature of the observables. This behaviour might be related to another question of mine that i asked here: Fpish

Thanks a lot an advance.