RelativeSource and Popup

The problem is that RelativeSource does not work in the following case. I use silverlight 5.

//From MainPage.xaml
<Grid x:Name="LayoutRoot" Background="White" Height="100" Width="200">
    <Popup IsOpen="True">
        <TextBlock Text="{Binding Path=DataContext, RelativeSource={RelativeSource AncestorType=Grid}}" />
    </Popup>
</Grid>

//From MainPage.xaml.cs
public MainPage()
{
    InitializeComponent();
    DataContext = "ololo";
}

If I set a breakpoint on the binding, I'll get Error:

System.Exception: BindingExpression_CannotFindAncestor.

If I use ElementName=LayoutRoot instead of RelativeSource, everything will be OK.

Why does the relative source binding not work?