Particles Additive adds black dot in the middle?

Hi, I have this simple plane (just 2 triangles) with a simple UV map (just a square), that works well in Diffuse (bottom in the image below). The texture is a TIFF from the official "Procedural Examples" project. When I change it to Particles Additive, a black dot appears exactly in the center pixel of the texture... What can be causing this? ![alt text][1] [1]: /storage/temp/11462-trontrail.png