This was the subject of horrendous trouble for me a while back.  On my old and new laptops, the argument can be either a string or a [string].  On my friend's ATI card, however, it MUST be a [string].  It was funny; it took months to track it down to that.  It was my fault really; I had edited it out in a very early version as it didn't seem to be be necessary...for me but not for the rest of the world, it turned out.