The simple answer is set the Transparency and Transparency_Edge dials to 1.0.

If you get weird shadow effects from the invisible material in your raytrace-enabled renders , it's because the transparency dials do not make the material transparent to raytracing.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)