Use LastOrDefaultAsync for completion.

src/Markup/Perspex.Markup/Data/ExpressionObserver.cs

@@ -197,11 +197,11 @@ namespace Perspex.Markup.Data
 
                 if (_rootObservable != null)
                 {
-                    source = source.TakeUntil(Complete(_rootObservable));
+                    source = source.TakeUntil(_rootObservable.LastOrDefaultAsync());
                 }
                 else if (_update != null)
                 {
-                    source = source.TakeUntil(Complete(_update));
+                    source = source.TakeUntil(_update.LastOrDefaultAsync());
                 }
 
                 var subscription = source.Subscribe(observer);
@@ -232,13 +232,6 @@ namespace Perspex.Markup.Data
             }
         }
 
-        private static IObservable<Unit> Complete<T>(IObservable<T> input)
-        {
-            return Observable.Merge(
-                input.TakeLast(1).Select(_ => Unit.Default),
-                input.IsEmpty().Where(x => x).Select(_ => Unit.Default));
-        }
-
         private void IncrementCount()
         {
             if (_count++ == 0 && _node != null)

src/Perspex.Styling/Styling/ActivatedObservable.cs

@@ -62,8 +62,8 @@ namespace Perspex.Styling
         {
             Contract.Requires<ArgumentNullException>(observer != null);
 
-            var sourceCompleted = Source.TakeLast(1).Select(_ => Unit.Default);
+            var sourceCompleted = Source.LastOrDefaultAsync().Select(_ => Unit.Default);
-            var activatorCompleted = Activator.TakeLast(1).Select(_ => Unit.Default);
+            var activatorCompleted = Activator.LastOrDefaultAsync().Select(_ => Unit.Default);
             var completed = sourceCompleted.Merge(activatorCompleted);
 
             return Activator