Troubleshooting Zebra_Form with password comparison

zf_compare.php

<?php
function ne_explorer_signup_form() {
  //ZF Library
	require_once(INCLUDES_DIR.'/lib/zebra_form/Zebra_Form.php');
	
	$form = new Zebra_Form('explorer-signup', 'POST');
	$form->add('label', 'label_firstname', 'explorer_firstname', 'Your name');
	$field = $form->add('text', 'explorer_firstname');
	$field->set_rule(array(
		'required' => array('error', 'Name is required'),
		'alphabet' => array('.,\'', 'error', 'Letters only please!'),
		'length' => array(2, 20, 'error', '20 character max'),
	));
	$field = $form->add('text', 'explorer_lastname');
	$field->set_rule(array(
		'required' => array('error', 'Name is required'),
		'alphabet' => array('.,\'', 'error', 'Letters only please!'),
		'length' => array(2, 20, 'error', '20 character max'),
	));
	$form->add('label', 'label_username', 'explorer_username', 'Pick a username');
	$field = $form->add('text', 'explorer_username');
	$field->set_rule(array(
		'required' => array('error', 'Username required'),
		'alphanumeric' => array('', 'error', 'Numbers and letters only!'),
		'length' => array(4, 20, 'error', 'Between 4 and 20 characters'),
	));
	$form->add('label', 'label_email', 'explorer_email', 'Email Address');
	$field = $form->add('text', 'explorer_email');
	$field->set_rule(array(
		'required' => array('error', 'Email address is required'),
		'email' => array('error', 'Not a valid email'),
	));
	$form->add('label', 'label_password', 'explorer_password', 'Choose a password');
	$field = $form->add('password', 'explorer_password');
	$field->set_rule(array(
		'required' => array('error', 'Password is required'),
		'length' => array(7, 0, 'error', 'More than 7 characters, please!'),
	));
	$form->add('label', 'label_password_confirm', 'explorer_password_confirm', 'Confirm your password');
	$field = $form->add('password', 'explorer_password_confirm');
	$field->set_rule(array(
		'compare' => array('explorer_password', 'error', 'Passwords must match!'),
	));
	$form->add('label', 'label_captcha', 'explorer_captcha', 'Spam Prevention');
	$field = $form->add('captcha', 'explorer_captcha');
	$field = $form->add('text', 'explorer_captcha_solution');
	$field->set_rule(array(
		'required' => array('error', 'Please prove you\'re not a robot!'),
		'captcha' => array('error', 'Please try again'),
	));
	$form->add('submit', 'explorer_submit', 'Create your profile');
	
	//Server side validation
	if( $form->validate() ) {
		//Form passed validation
		ne_process_explorer_signup($_POST);
	}
	
	//Show the form
	$form->render(INCLUDES_DIR.'/form_templates/form_explorersignup.php');
}
?>