Call to undefined function base_url() in the error_404 page

I'm starting with the version 3.0.0 of Codeigniter. One of the new features the manual defines that the error pages now is in the folder "view".  My intention was: customising the 404 error to use both localhost and the site. However, when I try to call the funciton base_url() in my error_404.php, I receive this message: "Call to undefined function base_url()";

How can I call the funcions of the helper Url in the error pages by default? Do I need to create a Controller to do it?

When I call the function show_404(); it works. For instance: http://www.jhonatanoliveira.com.br/a (if I inform the letter 'a', the Controller calls the show_404() function. Obs: I've modified the routes.php to allow it).

But in an incorrect url, an error appears: For instance: http://www.jhonatanoliveira.com.br/css/s...rap.min.cs