WPF throwing an exception parsing XAML that includes a Winforms User Control

A first chance exception of type 'System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll

Additional information:   is not a valid Win32 application. (Exception from HRESULT: 0x800700C1)  Error in markup file 'OsgViewer;component/osgviewerwin.xaml' Line 1 Position 9.

<Window x:Class="OsgViewer.OsgViewerWin"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:int="clr-namespace:System.Windows.Forms.Integration;assembly=WindowsFormsIntegration"
    xmlns:myns="clr-namespace:MyGlobalNS.MyNS;assembly=MyAssembly"
...
        <int:WindowsFormsHost x:Name="m_Host">
        	<myns:CMyClass  x:Name="m_MyClass" />
    	</int:WindowsFormsHost>
...
</window>