Binding Geometry properties in an ItemsControl

In a Silverlight game I'm working on I'm using an ItemsControl to display an ObservableCollection of game objects that we'll call Foo. Foo implements INotifyPropertyChanged and has a single property: Radius. The ItemsControl's ItemTemplate represents each Foo as an circular path, with the radius of the path bound to Foo.Radius.

The problem I'm running into is that whenever I try to add something to the ObservableCollection I get an InvalidOperationException with the message "Operation is not valid due to the current state of the object." If a remove the RadiusX and RadiusY bindings program runs fine, and it still works if I bind Foo.Radius to some prop开发者_运维百科erty of Path. I'm at a loss for how to bind the geometry properties. Am I missing something?

XAML for reference:

<ItemsControl ItemsSource="{Binding}" x:Name="LayoutRoot">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Path Stroke="Black">
                <Path.Data>
                    <EllipseGeometry RadiusX="{Binding Radius}" RadiusY="{Binding Radius}" />
                </Path.Data>
            </Path>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

Codebehind:

private ObservableCollection<Foo> things = new ObservableCollection<Foo>();

public MainPage()
{
    InitializeComponent();
    LayoutRoot.DataContext = things;
    CompositionTarget.Rendering += Update;
}

void Update(object sender, EventArgs e)
{
    things.Add(new Foo());
}

Try this ...

   Dispatcher.BeginInvoke(() => things.Add(new Foo()));

I did a bit more searching and discovered that in Silverlight 3 it's only possible to bind properties to FrameworkElements, but Geometry inherits from DependencyObject. Upgrading the project to Silverlight 4 seemed to fix the problem.