Problem with transparent image

Hi everyone,

I'm having problem with embedding transparent image to PDF. Thin border appears  when viewing the document with Adobe Reader, Adobe Acrobat Pro and even with Foxit Reader. But there are no such border in the image.

After some experiments I found that this border appears on the edge of fully transparent black and fully opaque white areas. And changing fully transparent black color to fully transparent white removes the border.

Is this an expected behaviour of Adobe Reader or is it a bug? Could anyone explain in which other cases such artifacts may appear?

Thanks in advance.