Magick::Pixel#opacity randomly returns 0 or 65535

[lesniakania]

According to http://www.imagemagick.org/RMagick/doc/struct.html#Pixel, opacity should be 0 for visible pixels and (not mentioned there, but found on the internet) 65535 for fully transparent pixel.

I found out that for some images it works randomly - most of the time returns 0, but sometimes 65535.
On this image: https://www.dropbox.com/s/uo5ejfywr5uwkqk/sample3.png?dl=0 it happens for black (rgb(0, 0, 0)) pixel.

I found out it on Ubuntu 14.04, I was not aware about this while working on my local OSX (10.9.4)
ImageMagick version is: 6.7.7.