diff --git a/Rx.NET/Source/src/System.Reactive/Linq/Observable/GroupByUntil.cs b/Rx.NET/Source/src/System.Reactive/Linq/Observable/GroupByUntil.cs
index 0e48e92be2..cec104de22 100644
--- a/Rx.NET/Source/src/System.Reactive/Linq/Observable/GroupByUntil.cs
+++ b/Rx.NET/Source/src/System.Reactive/Linq/Observable/GroupByUntil.cs
@@ -141,9 +141,9 @@ public override void OnNext(TSource value)
                     lock (_gate)
                         ForwardOnNext(group);
 
-                    var md = new SingleAssignmentDisposable();
-                    _groupDisposable.Add(md);
-                    md.Disposable = duration.SubscribeSafe(new DurationObserver(this, key, writer, md));
+                    var durationObserver = new DurationObserver(this, key, writer);
+                    _groupDisposable.Add(durationObserver);
+                    durationObserver.SetResource(duration.SubscribeSafe(durationObserver));
                 }
 
                 var element = default(TElement);
@@ -178,33 +178,31 @@ public override void OnNext(TSource value)
                 writer.OnNext(element);
             }
 
-            private sealed class DurationObserver : IObserver<TDuration>
+            private sealed class DurationObserver : SafeObserver<TDuration>
             {
                 private readonly _ _parent;
                 private readonly TKey _key;
                 private readonly ISubject<TElement> _writer;
-                private readonly IDisposable _self;
 
-                public DurationObserver(_ parent, TKey key, ISubject<TElement> writer, IDisposable self)
+                public DurationObserver(_ parent, TKey key, ISubject<TElement> writer)
                 {
                     _parent = parent;
                     _key = key;
                     _writer = writer;
-                    _self = self;
                 }
 
-                public void OnNext(TDuration value)
+                public override void OnNext(TDuration value)
                 {
                     OnCompleted();
                 }
 
-                public void OnError(Exception error)
+                public override void OnError(Exception error)
                 {
                     _parent.Error(error);
-                    _self.Dispose();
+                    Dispose();
                 }
 
-                public void OnCompleted()
+                public override void OnCompleted()
                 {
                     if (_key == null)
                     {
@@ -225,7 +223,7 @@ public void OnCompleted()
                         }
                     }
 
-                    _parent._groupDisposable.Remove(_self);
+                    _parent._groupDisposable.Remove(this);
                 }
             }