!is_null() not working as expected

[mirc022]

\[quote\] dispatch_address_postcode\[/quote\]isn't mandatory and it will still run even if it's blank:\[code\]if (!is_null($_POST['personal_info_first_name']) && !is_null($_POST['personal_info_surname']) && !is_null($_POST['personal_info_email']) && !is_null($_POST['personal_info_telephone']) && !is_null($_POST['dispatch_address_country']) && !is_null($_POST['dispatch_address_first_name']) && !is_null($_POST['dispatch_address_surname']) && !is_null($_POST['dispatch_address_address']) && !is_null($_POST['dispatch_address_town']) && !is_null($_POST['dispatch_address_postcode']) && !is_null($_POST['dispatch_address_county']) && ( ($_POST['payment_method'] == "Pay by credit card.") || ( ($_POST['payment_method'] == "Pay by new credit card.") && !is_null($_POST['card_number']) && !is_null($_POST['expiration_date']) && !is_null($_POST['security_code']) ) ))\[/code\]What gives?