ParticleAnalyzer question

[Ludgate, David]

Hi,
I've extended the ParticleAnalyzer class to give me an ArrayList of
ByteProcessor objects corresponding to all the particles found. I did
this simply by overriding the drawParticle method (since I have no need
of it anyway). When I get it working I'll look for a more elegent
method, but this was simple and works. The problem is, I often get null
values in my ArrayList. What would cause the ParticleAnalyzer to try to
draw particles with a null mask?
Here is my method.
 
 protected void drawParticle(ImageProcessor drawIP, Roi roi,
   ImageStatistics stats, ImageProcessor mask){
  procList.add(mask);
 }
 
thanks!
dave