WPF: Binding Error, obwohl es funktioniert!?

Hallo Leute,

folgendes Control:

public class ViewItemBase : Control, INotifyPropertyChanged
	{
		static ViewItemBase()
		{
			FrameworkElement.DefaultStyleKeyProperty.OverrideMetadata(
				typeof(ViewItemBase), new FrameworkPropertyMetadata(typeof(ViewItemBase)));
		}

....

	#region RotationAngle

		public double RotationAngle
		{
			get { return (double)GetValue(RotationAngleProperty); }
			set { SetValue(RotationAngleProperty, value); }
		}

		// Using a DependencyProperty as the backing store for RotationAngle.  This enables animation, styling, binding, etc...
		public static readonly DependencyProperty RotationAngleProperty =
			DependencyProperty.Register("RotationAngle", typeof(double), typeof(ViewItemBase), new UIPropertyMetadata(0.0, OnRotationChange));

		private static void OnRotationChange(DependencyObject d, DependencyPropertyChangedEventArgs e)
		{
			var me = d as ViewItemBase;
			me.CalcBounds();
		}

		#endregion
...
}

der XAML Style dazu:

<Style x:Key="{x:Type Model:ViewItemBase}" TargetType="{x:Type Model:ViewItemBase}">
		<Setter Property="RenderTransformOrigin" Value="0.5,0.5"/>
		<Setter Property="SnapsToDevicePixels" Value="true"/>

		<Setter Property="LayoutTransform">
			<Setter.Value>
				<RotateTransform Angle="{Binding RotationAngle, RelativeSource={RelativeSource AncestorType={x:Type Model:ViewItemBase}}}"/>
			</Setter.Value>
		</Setter>
	</Style>

Das Binding des RotationTransform Angle funktioniert, aber trotzdem bekomme ich ein Binding error:

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='Designer.Core.Controls.Model.ViewItemBase', AncestorLevel='1''. BindingExpression:Path=RotationAngle; DataItem=null; target element is 'RotateTransform' (HashCode=62225447); target property is 'Angle' (type 'Double')

Was is da falsch ! ?? Obwohl es geht:)

P.S. Alternativ RelativeSource={RelativeSource TemplateParent} geht nicht,deswegen über die Ancester!!

grüße