WidgetSet optimisation exception


I was trying to accomplish the widget set optimisation on my app following the tutorial:

I compile the widgetset via a Maven profile.
At the compilation I’m getting the following exeption:

A generator must extend com.google.gwt.core.ext.Generator
Line 21: Unexpected exception while processing element ‘generate-with’

my loader focatory is as follows:

public class OptimizedConnectorBundleLoaderFactory extends
ConnectorBundleLoaderFactory {

Anyone got this thing?

I use Vaadin 7.1.9.