Gravity Forms Coupon Code Validation

<?php
// change 99 to the ID of the Discount Code field
add_filter("gform_field_validation_99", "custom_validation", 10, 4);
function custom_validation($result, $value, $form, $field){
    $coupon_codes = array(
    'NEWCUST20',
    'RENEWCUST5',
    'FREEFRIDAY'
    );
    $value = strtoupper($value); // make sure we are comparing apples to apples
    if (!in_array($value,$coupon_codes){
        $result['is_valid'] = false;
        $result['message'] = 'Invalid code.';
    }
    return $result;
}
?>