Focus() Method Doesn't Work

<Window 
    x:Class="Sample.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Editors="clr-namespace:ActiproSoftware.Windows.Controls.Editors;assembly=ActiproSoftware.Editors.Wpf30"
    Title="Window1"
    Width="600"
    Height="300"
    >
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>

        <Editors:Int32EditBox
            x:Name="uxTextBoxOne"
            Margin="3"
            Width="100"
            Height="21"
            />
        <Editors:Int32EditBox
            Margin="3"
            Grid.Row="1"
            Width="100"
            Height="21"
            />
        <Button
            Grid.Row="2"
            Content="Set Focus"
            Click="Button_Click"
            />
    </Grid>
</Window>

namespace Sample
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, System.Windows.RoutedEventArgs e)
        {
            uxTextBoxOne.Focus();
        }
    }
}