request.getWrappedSession(true) returned null?!?

I find that, although I request the creation of a session, I'm getting null:

WrappedSession ses = request.getWrappedSession(true);  // true --> create one if none existing
assert ses != null;// explodes!

This code is being called by my login button event handler.    The request is the the one that was given to me when I created the GUI:  init(VaadinRequest request);   Should I be getting a new request in the button event handler? 

Perhaps a bug or perhaps my misunderstanding (please extend the documentation if so).

Anyway, thought you'd like to know.  I'm avoiding it by this instead:

ses = VaadinSession.getCurrent().getSession();

Details:
vaadin: 7.5.7
client browser: Chrome 44 (latest)
client OS: Windows 7 Enterprise
server OS: Windows 7 Enterprise
servlet container: Tomcat 8 (latest) on Sun JRE 1.8.0_60 (latest)