Object in an alpha image has the world color around its edge

Topic says it all.
Under render, I select .png and use RGBA so that the image has alpha supported.
But the object has the world background color blended around its edge. (see attachment)
Does anybody know how to solve this? I want pure transparency around its edge, not the background color…

EDIT : The image didn’t upload with transparency, but all the black except for the sphere in the middle is supposed to be transparent…

You can select Premultiply in the Render / Shading / Alpha dropdown

That did the trick, thanks Richard.