How do I use INotifyPropertyChanged to update an array binding?

Let's say I have a class:

class Foo
{
  public string Bar
  {
    get { ... }
  }

  public string this[int index]
  {
    get { ... }
  }
}

I can bind to these two properties using "{Binding Path=Bar}" and "{Binding Path=[x]}". Fine.

Now let's say I want to implement INotifyPropertyChanged:

class Foo : INotifyPropertyChanged
{
  public string Bar
  {
    get { ... }
    set
    {
      ...

      if( PropertyChanged != null )
      {
        PropertyChanged( this, new PropertyChangedEventArgs( "Bar" ) );
      }
    }
  }

  public string this[int index]
  {
    get { ... }
    set
    {
      ...

      if( PropertyChanged != null )
      {
        PropertyChanged( this, new PropertyChangedEventArgs( "????" ) );
      }
    }
  }

  public event PropertyChangedEventHandler PropertyChanged;
}

What goes in the part marked ????? (I've tried string.Format("[{0}]", index) and it doesn't work). Is this a bug in WPF, is there an alternative syntax, or is it simply that INotifyPropertyChanged isn't as powerful as normal binding?