Diffuse pass problem

Hi. In the node editor when I want to put a antialiased object over a background I use “alpha over” node with premul enabled, it works fine (witout artifacts around the borders) when I use the whole image, the combination of all passes . But when I want to put only the diffuse pass over a background artifacts apper even with multiplying alpha in advance.

here is the blend file


Thanks, I have found a another way, I use a combined channel “image” with very low specularity settings on my materials instead a diffuse channel, and when I need some specularity I use a color mix node set to add with a very high factor.