render_as_string behaviour

Hello guys, I tried to use rtex to generate pdfs, and it worked fine. Then I tried to use render_as_string to I could have the pdf data and put it somewhere (specific) in my hard drive, but for some unknown reason (at least for me :D) the result of render_as_string is always (if the action is valid) nil... I'm using rails 2.0.2

Any ideas?

Tks in advance, jose