No... you haven't got me correctly. Method is gonna always exist on `Request` object.
The problem is that if the argument exists, it returns it. Otherwise, it throws an `InvalidArgumentException`.
Look at the implementation of method `Request::getTrustedHeaderName`:

```php
    public static function getTrustedHeaderName($key)
    {
        if (!array_key_exists($key, self::$trustedHeaders)) {
            throw new \InvalidArgumentException(sprintf('Unable to get the trusted header name for key "%s".', $key));
        }

        return self::$trustedHeaders[$key];
    }
```