Call to a member function real_escape_string() on a non-object

When I use the load->database with the seccond param TRUE, I get the error

Code:

Severity: Error

Message: Call to a member function real_escape_string() on a non-object

Filename: mysqli/mysqli_driver.php

Line Number: 321

This happened after upgrade from codeigniter 2 to 3. I cannot find the problem in the upgrading guide.

The code I use is:

PHP Code:

$ci =& get_instance();
$this->userdb = $ci->load->database('default', TRUE);

$qres = $this->userdb->get($this->table_name);