Dialogresult work if button click but not on keydown

Hi,

Have a problem with dialogresult in a WPF form.

The form is started from main method (static void Main).

Show dialog window start:

new WindowStart().ShowDialog(); 

In window start, this works:

private void button1_Click(object sender, RoutedEventArgs e)
{
       DialogResult = true;
}

But not this:

        void Window_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Key == Key.Enter)
            {
                DialogResult = true;
            }                
        }

It throws a InvalidOperationException: DialogResult can be set only after Window is created and shown as dialog.

Why does dialogresult work on button click but not on keydown?