Beta 2 DataTemplate Error

Using Blend 2.5 June and SL 2 Beta 2, I created a DataTemplate using Blend:

  <vsm:DataTemplate x:Key="DataTemplate1">
   <StackPanel Background="#FFE6E8F1">
    <StackPanel Orientation="Horizontal">
     <TextBlock Text="{Binding Sender}" FontSize="18"  />
     <TextBlock Text="{Binding Date}" FontSize="12"  Margin="110,5,0,0"/>
    </StackPanel>
    <TextBlock Text="{Binding Subject}" FontSize="14" />
    <TextBlock Text="{Binding Text}" TextWrapping="Wrap"  />
   </StackPanel>
  </vsm:DataTemplate>


But when I run the app, I get a runtime error th ...