Express flash messages with passport doesn't work

Folloing the sample http://ift.tt/1wF0QPo I have create a simply signup app to test passport.

I'm using Express 4 and connect-flash module.

My routes are as follow, that is on GET /signup I show the signup form with any possible signupMessage flash message (always undefined). On POST /signup I try to authenticate credentials:

router.get('/signup', function(req, res) {
  debug('GET signup flash message "%s"',req.flash('signupMessage'));
  res.render('signup',  { message: req.flash('signupMessage') });
});

// process the signup form
router.post('/signup', passport.authenticate('local-signup', {
  successRedirect : '/profile', // redirect to the secure profile section
  failureRedirect : '/signup', // redirect back to the signup page if there is an error
  failureFlash : true // allow flash messages
}));

My passport related code is place next. It works but the lines like return done(null, false, req.flash('signupMessage', 'That email is already taken.')); seems that must set the request flash message seems not work because the flash message is never shown in the signup form shown by the GET method.

passport.use('local-signup', new LocalStrategy({
    // by default, local strategy uses username and password, we will override with email
    usernameField : 'email',
    passwordField : 'password',
    passReqToCallback : true // allows us to pass back the entire request to the callback
},
function(req, email, password, done) {

    debug("Auth");

    // asynchronous
    // User.findOne wont fire unless data is sent back
    process.nextTick(function() {

    debug("before findOne");

      // find a user whose email is the same as the forms email
      // we are checking to see if the user trying to login already exists
      User.findOne({ 'local.email' :  email }, function(err, user) {

        debug("inside findOne");

          // if there are any errors, return the error
          if (err) {
            return done(err);
          }

          // check to see if theres already a user with that email
          if (user) {
            debug('user exists');
            return done(null, false, req.flash('signupMessage', 'That email is already taken.'));
          } else {

            debug('creating new user');

            // if there is no user with that email
            // create the user
            var newUser            = new User();

            // set the user's local credentials
            newUser.local.email    = email;
            newUser.local.password = newUser.generateHash(password);

            // save the user
            newUser.save(function(err) {
              if (err) {
                throw err;
              }
              return done(null, newUser);
            });
          }

      });    

    });

}));

After looking for help I also try to put:

return done(null, false, {message: 'That email is already taken.'});

and in the GET /signup method use:

router.get('/signup', function(req, res) {
  res.render('signup',  { message: req.flash('error') });
});

but that doesn't works.

Anybody can help me?

Thanks.