> Also, making subsequent calls to command.CanExecuteObservable will return null.

Ah, yes, forgot to add that test, that's actually ht eproblem I ran into.

I have an Android App, and what I do is binding a command to a button

`this.BindCommand(this.ViewModel, x => x.TheCommand, x => x.theButton)`

and then, depending whether the command can be executed or not, I set an image as background

```csharp
this.ViewModel.TheCommand.CanExecuteObservable.Select(x => /* select image */)
   Subscribe(x => this.TheButton.SetBackgroundResource(x));
```

This fails with a `NullReferenceException`