How do I call the correct form field title for this this alter_form function I created?

I created a form to do some custom validation on the Postal_Code field that Location provides. The code is as follows

<?php
function location_extra_node_validate($node, $form, &$form_state) {
	//dsm($form);
	//print "<pre>";
	//print_r($form['field_item_location']['und'][0]['postal_code']);
	//print_r($form_state['values']['field_item_location']['und'][0]['postal_code']);
	//print "</pre>";
	
	// create a variable that is a little easier to work with
	$zip = $form_state['values']['field_item_location']['und'][0]['postal_code'];
	
	// let's check to see if the value is a 5 digit US zipcode
	if($match = preg_match("#[0-9]{5}#", $zip)) {
		// check to see if the zipcode is actually in the database
		$result = db_query('SELECT zip FROM zipcodes WHERE zip = :zip', array(':zip' => $zip));
		
		//count to see if result is 0, indicating an invalid zipcode
		$number_of_rows = $result->rowCount();
		
		//if $number_of_rows is 0, throw form_set_error()
		if($number_of_rows == 0) {
			form_set_error('postal_code', 'Your zip code was not found. Please make sure you entered a valid 5 digit zip US code.');
		}
	} else {
		form_set_error('postal_code', 'This is not a valid zip code. Please enter a 5 digit US zip code.');
	}
}
?>

The function works, but when the error is thrown by form_set_error, the postal_code form field is not highlighted. I am sure it's because I have the wrong argument 'postal_code.' Can someone tell me what the correct title is so that highlighting works correctly on an error?

Thanks