Exception occurs when there are no items in the collection and load on demand mode is automatic:
public ViewModel()
{
this.Source = new ObservableCollection<string>();
//for (int i = 0; i < 14; i++)
//{
// this.Source.Add(string.Format("Item {0}", i));
//}
this.LoadItemsCommand = new Command(this.LoadItemsCommandExecute);
}