New Post: MessageBox Error on .Show()

I'm attempting to use the Xceed Toolkit MessageBox. When Show is called, I get a XamlParseException. The message is that "DependencyProperty.UnsetValue is not a valid value for property 'BorderBrush'.

Any ideas why this would be happening? I've check my Theme and insured that I have a style created for the messagebox with valid references to BorderBrush resources.

EDIT: So, I guess part of my problem was the ordering of the resources in the theme file. I've changed the order to have the border declared before the MessageBox and that fixed part of the problem. However, now I'm showing the same error for the "CloseButtonStyle". This time though, it is definitely declared prior to the MessageBox style.