Problem with Visual Brush

I am trying to use a visual brush to create a mirror effect, but I'm
getting a binding error:

System.Windows.Data Error: 4 : Cannot find source for binding with
reference 'ElementName=frontPanelBorder'. BindingExpressionno path);
DataItem=null; target element is 'VisualBrush' (HashCode=54814357);
target property is 'Visual' (type 'Visual')

The interesting bit is that the element "frontPanelBorder" is defined in
the same XAML file, and the visual brush works when I'm in design mode
in Blend (I can see the proper reflection)- the error only occurs when I
compile and run my app.

Any ideas?

Thanks,
--jeremy