Navigation With Button !!!

Hi All.
I am trying the open a Window wtih a button.
My codes-->>
private void button1_Click(object sender, RoutedEventArgs e)
{
_myFrame.Navigate(myUri);
}


"And I tried <_myFrame.NavigationService.Navigate(myUri)>  too ."

But both of them catches Error .   Error : " InvalidOperationException  The root element is not valid for navigation."

I wanted to use this.NavigationService but i couldnt find it. when I write this. there is no NavigationService in the menu ..

Thanks..