Why do I get a DependencyProperty.UnsetValue when converting a value in a MultiBinding?

From MSDN:

UnsetValue is a sentinel value that is used for scenarios where the WPF property system is unable to determine a requested DependencyProperty value. UnsetValue is used rather than null reference (Nothing in Visual Basic), because null reference could be a valid property value, as well as a valid (and frequently used) DefaultValue.

Which means one of the following things:

• You use a template (ControlTemplate or DataTemplate), and the value does not have a DataSource set at the time of being Loaded. So it will hit your converter twice, first with the UnsetValue, second with the boolean value; so nothing to worry about;
• Your Binding is incorrect, meaning the Binding cannot determine a value, thus resulting in the UnsetValue.. You should propbably see a warning..

Also, you cannot combine Converters like you do.. So its probably that.

Remove the Converter in the inner Binding, and it should be fixed! 🙂

Hope this helps!