AutoCompleteBox cannot set focus on load

Hi,

I have a window with the autocompletebox and I'm trying to set the focus to it to it so the user can type right away. While it works on any control such as textbox it doesn't work on the autocompletebox.

 public TestWindow()
        {
            DataContext = this;
            InitializeComponent();
            autoCompleteBox.Text = "test";
            autoCompleteBox.Focus();
        }

 I always have to press TAB once to set the focus after the window has been created. Is there another way to set the focus to it?