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,