Selecting first item in listbox

A listbox works as an auto-complete within a richtextbox I am populating it with items from a collection. I need it to auto select first item every time listbox populates.

How do I do this?

Thank you

foreach (var ks in ksd.FindValues(comparable))
      {
          lb.Items.Add(ks.Value);
      }

      if (lb.HasItems)
      {
          lb.Visibility = System.Windows.Visibility.Visible;
          lb.SelectedIndex = 0; //Suggested solution, still doesn't work 
      }
      else
      {
          lb.Visibility = System.Windows.Visibility.Collapsed;
      }