Form validation with array as value

[eluser]Unknown[/eluser]
This was working in previous CI version. Not sure if I am missing something, but I cannot get this to validate when the value of the post is an array...

Code:

$_POST['field[]'] = array('2', '3', '5');

$this->load->library('form_validation');
$val = $this->form_validation;

$val->set_rules('field[]', 'FIELD', 'required');

if($val->run()) echo set_value('field[]');
else echo 'ERROR:'.form_error('field[]');