wpf - Silverlight 3: Event trigger argues that attribute is out of range -

When I enter the mouse I want to make the grid transparent. So I try to capture the MouseEnter event and create adverse assets

   & Lt; / Storyboard & gt; & Lt; / BeginStoryboard & gt; & Lt; / EventTrigger & gt; & Lt; /Grid.Triggers> & Lt; / Grid & gt;   
 But InitializeComponent argues that the Grid.MouseEnter attribute is out of bounds, am I wrong?   
 
 
 Your code works here.  
 I use Visual Studio 2010 and paste my code into a C # WPF application.