Problem with System.Windows.Controls in Moonlight

I can't seem to use a GridSplitter in my Xaml:

<basics:GridSplitter ResizeDirection="Columns"
Width="5"
Grid.Column="0"
Grid.ColumnSpan="1"
Grid.Row="1"
Grid.RowSpan="3"
Grid.IsSharedSizeScope="True"
HorizontalAlignment="Right"
VerticalAlignment="Stretch">
</basics:GridSplitter>

I have the namespace defined:
xmlns:basics="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls"

and I have a reference to "System.Windows.Controls" set.

When I debug the app, I get this error:
Could not parse element GridSplitter, attribute (null), error: Unable to resolve managed type GridSplitter.

What am I doing wrong?