get_instance error - Class 'CI_Controller' not found

In my "helper" function in the "application" folder,

PHP Code:

if ( ! function_exists('_return_config_xxx'))
{
    function _return_config_xxx(){

        $CI =& get_instance();
    return $CI->config->item('config_xxx')
    }
} 


I call :
$ CI = & get_instance ();
return $ CI-> config-> item ('XXX');

and I get the following error:

Code:

Fatal error: Class 'CI_Controller' not found in /home/www/aprendermais.render.com.br/html/system/core/CodeIgniter.php on line 369
A PHP Error was encountered

Severity: Error

Message: Class 'CI_Controller' not found

Filename: core/CodeIgniter.php

Line Number: 369

Backtrace: