Captcha and Akismet addons don't work together

[posefius]

I'm using Akismet for a long time and I'm very satisfied with the good working of it. It stops about 100 SPAM's every day. Now I decided to use Captcha also to reduce the amount of comment SPAM. I tried to let both addons work together but they don't. When I disable the include of the Akismet addon in the index.php the Captcha works fine, but when I enable it Captcha stops working.

//include the Akismet comment filter script *after* Pixelpost's own moderation
// include('addons/admin_akismet_comment.php');

I debugged it a bit and figured out that the $userinput field with the entered Captcha code is empty when the audit function is called (anti_comment_spam.php).

Does anyone have a fix for this problem?