Drawing anti-aliased text with transparent background

Gives me this resut:
http://www.greenpixeldesign.com/cphandler/tmp-img/test.png

I am using libgd 2.0.33 on debian, so basically I just create an image from
scratch, set transparent color to black and use gdImageStringFT to draw to
text. (I use lisp bindings to do that). What am I missing to remove the
black outline from the text?

Thank you,
Andrew