The documentation for IValueConverter.ConvertBack recommends returning DependencyProperty.UnsetValue.
The data binding engine does not catch
exceptions that are thrown by a
user-supplied converter. Any exception
that is thrown by the ConvertBack
method, or any uncaught exceptions
that are thrown by methods that the
ConvertBack method calls, are treated
as run-time errors. Handle anticipated
problems by returning
DependencyProperty.UnsetValue.