Wouldn't it be better to raise an exception when an unsupported type is passed:
```php
$idTypes = array('ASIN', 'SKU', 'UPC', 'EAN', 'ISBN');
if (!in_array($idType, $idTypes)) {
    $msg = "Invalid type '%s' passed. Valid types are: '%s'");
    throw new \InvalidArgumentException(sprintf($msg, $idType, $idTypes));
}
```