Mongoose population call (maybe?) hanging

I'm following the thinkster.io MEAN tutorial - everything works, but I tried to go off-piste a little, and I don't understand why my changes don't work. After my changes, GETting /posts/<id> simply never responds - node logs the request, but no response is sent.

Working code

router.param('post', function(req, res, next, id) {
  var query = Post.findById(id);

  query.exec(function (err, post){
    if (err) { return next(err); }
    if (!post) { return next(new Error('can\'t find post')); }
    req.post = post;
    return next();
  });
});

router.get('/posts/:post', function(req, res, next) {
  req.post.populate('comments', function(err, post) {
    if (err) { return next(err); }

    res.json(post);
  });
});

Broken code

All I've done is move the populate call up to the parameter handler (is that the correct term?) - to my mind, that should work - by the time the GET handler comes around, req.post should have it's comments populated, no?

router.param('post', function(req, res, next, id) {
  var query = Post.findById(id);

  query.exec(function (err, post){
    if (err) { return next(err); }
    if (!post) { return next(new Error('can\'t find post')); }

    post.populate('comments', function(err, spost) {
        if (err) { return next(err); }
        req.post = spost;
    });

    return next();
  });
});

router.get('/posts/:post', function(req, res) {
  res.json(req.post);
});