Telerik RadWindow Blend Issue

The following code throws an exception: "Error HRESULT E_FAIL has been returned from a call to a COM component."

<telerik:RadWindow 
	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	xmlns:telerik="clr-namespace:Telerik.Windows.Controls;assembly=Telerik.Windows.Controls.Navigation" 
	mc:Ignorable="d"
	x:Class="Sample.UserControl1"
	d:DesignWidth="640" d:DesignHeight="480">

	<Grid x:Name="LayoutRoot"/>
</telerik:RadWindow>