Rename a field with dbforge

I'm trying to rename a field, like this:

PHP Code:

$this->load->dbforge();
$fields = array(
    'old_name' => array(
        'name' => 'new_name',
        'type' => 'VARCHAR'
    )
);
$this->dbforge->modify_column('table', $fields); 


This gives an error message: MySQL syntax error on line 2, near ''
I can't see what I'm doing wrong here. Any help is appreciated.