how to invalidate a user session properly?

[Yucca Nel]

Currently I need to fire my jsf command button twice to get the logic behind it 
to invalidate the session correctly as follows:
 public String logout(ActionEvent ae) throws IOException, ServletException {
        HttpServletRequest req= (HttpServletRequest) 
FacesContext.getCurrentInstance().getExternalContext().getRequest();
        req.getSession(false).invalidate();

        return "home";

the backing bean is request scope and I though that that may have been the 
issues. I would like to add that simply changing the mthod to one for a simple 
action and not actionListener makes no difference.

Here is where the button is called...

<h:commandButton rendered="#{request.userPrincipal!=null}" 
actionListener="#{logoutForm.logout}" 
value="#{uOptMsg.logOut}"></h:commandButton>

tomcat 6 being used.