Include function from template.php

Hi

I have a defined function inside template.php wich I'd like to use inside computefield. The function works perfectly when I call it from my templates (i.e. node.tpl.php) but it doesn't work at all when I call it from computed field. The weird issue for me (wich I don't know much PHP) is it doesn't return a "called to unkown function error" but just an empty return.

Where should I call this function?

Thanks in advance.