issue compositing with alpha channel

Veblen

The first image is a reference render, the second is the center sphere rendered in a node list on a beauty canvas with alpha, composited over a background render using the script below. In the second image there is a gray halo. The foreground RGB values were premuilitplied when the image was generated, so I'm not sure what's going on.

 

CompositeLayers: function (files, outputFileName) {	var outImage;	files.forEach(function (file) {		if (!outImage)			outImage = new Image(file);		else {			var currentImage = new Image(file);			outImage = currentImage.compositeOver(outImage);		}	});	var outPixmap = new Pixmap();	outPixmap.fromImage(outImage);	outPixmap.save(outputFileName);}