Order of session_regenerate_id()

[sidsel]

I am doing a login script using sessions and a CAPTCHA image.

If login is successfull I do the following:

$_SESSION['captcha_string'] = "";
session_regenerate_id();

But in the old session, $_SESSION['captcha_string'] still contains the captcha string even though I reset it before generating a new session id...

Only the new session contains a empty captcha string. How can it be?