res.locals returned undefined

I'm trying to set some response-specific variables, and I'm getting undefined for res.locals when I log it from within my middleware, but it returns the function just fine if I log it from within a route function.

// all environments
app.set('port', process.env.PORT || 3000);
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');
app.use(express.favicon());
app.use(express.logger('dev'));
app.use(express.json());
app.use(express.urlencoded());
app.use(express.methodOverride());
app.use(function (res, req, next) {
  console.log("res.locals from app.use middleware: ", res.locals);
  // res.locals.boom = 'nice';
  // res.locals.zoom = 'yeah!';
  next();
});
app.use(app.router);
app.use(require('stylus').middleware(path.join(__dirname, 'public')));
app.use(express.static(path.join(__dirname, 'public')));

Any ideas?