Opera redirecting page with previous anchor

Question

Hi - here's what I have:

index.php

<form method="post" action="index.php#error_message">
...

If form submission validates, at the top of index.php, I have:

<?php
...
header('Location: thanks.php#thanks_message');
...

So, by the time it gets to the thanks.php page, it should anchor to #thanks_message. Works fine in IE, Firefox, Chrome and Safari. However, in Opera, the resulting URL is:

http://.../thanks.php#error_message

Why does Opera maintain the old anchor (#error_message) even after a redirect? The only way it can propagate is through the <form action...>.

Is this a bug in Opera?

Thanks.