transitionToRoute returns error in route

I'm retrieving posts from the store using store.find. When nothing could be found my server I'd like to transition to a different route but transitionToRoute() gives this error:

Error while processing route: posts undefined is not a function TypeError: undefined is not a function

When nothing could be found my server return HTTP 200 and the following JSON

{
    "posts": [],
}

This is the route/posts.js

import Ember from 'ember';

export default Ember.Route.extend({

  model: function(params) {

    var self = this
    return this.store.find('posts', { permalink: params.permalink}).then(function(posts) {
      // Error while processing route: base.posts undefined is not a function TypeError: undefined is not a function
      if (posts.get('length') === 0) {
        self.transitionToRoute('not-found')
      }
      return posts
      );
  },

  afterModel: function(posts, transition) {
    if (posts.get('length') === 0) {
      // same error
      this.transitionTo('not-found');
    }
  }
});