Ember.js - Error while processing route if URL hit directly

I keep getting the following error from the route handler for a single object (user). If I click the link to view/show a single object (user) it works fine, but when I refresh the page at this route's url I keep getting this error.

URL that causes the error when visiting directly:

/users/:user_id

Error message in console:

Error while processing route: users.show Assertion Failed: Expected an
object as `data` in a call to `push`/`update` for ember-app@model:user:,
but was undefined Error: Assertion Failed: Expected an object as `data` 
in a call to `push`/`update` for ember-app@model:user: , but was undefined

app/router.js

Router.map(function() {
    this.resource('users', function() {
        this.route('show', { path: '/:user_id' });
    });
});

app/routes/users/show.js

export default Ember.Route.extend({
    model: function(params) {
        console.log('show route model hook'); //Hook firing on refresh
        return this.store.find('user', params.user_id);
    }
});

I am using Ember 1.8.1 and Ember-Data 1.0.0-beta.12. Any help will be greatly appreciated!