Strange behavior with mongoose, express and EJS

This is the mongoose and express code:

app.get('/:user/:id', function (req, res){

   usermodel.findOne({ user: req.params.user }, function (err, user){

   var imagen = user.imagen.id(req.params.id);

   console.log(imagen);

    res.render('photo.ejs', {

         user: user,
         photo: imagen

    });
   });

});

And this is the output in the EJS template:

<img src="/users/<%= user._id %>/<%= user.avatar %>" class="img-circle" alt="">

I've a console.log, returning the imagen object. Just once. But I receive this:

{ title: 'Slimes',
  name: 'slimes.png',
  author: 'Mrmangado',
  description: 'A collection of Slimes',
  index: 1,
  views: 7,
  fav: 0,
  path: '/users/5112b8da8a63aae76f000005/',
  _id: 5112b91a8a63aae76f000006,
  comments: [],
  date: Wed Feb 06 2013 23:12:10 GMT+0300 (MSK),
  tags: [ '#Slimes', '#DQ', '#Sprites' ] }
null
TypeError: /root/views/photo.ejs:92

  >> 1|       <img src="/users/<%= user._id %>/<%= photo.name %>" alt=""/>

I get the imagen object twice, the first is defined, but the second time has a value of null, so I get an error. Why? It's really weird why I get two request, maybe, to that URL. Any solution for this...?

Thank's advance!