"Prefix free" isn't pre-fixing image filters...?

Question

I'm trying to use "Prefix free" (http://leaverou.github.io/prefixfree/) on image filters and it seems the script isn't adding a -webkit- prefix to it.

Here's the example: http://codepen.io/barkins/pen/ByeGjd

img {
  filter: sepia(.5);
}

Shouldn't "Prefix free" add "-webkit- to the style? So it would look something like this:

img {
  filter: sepia(.5);
  -webkit-filter: sepia(.5);
}

Thanks advance for any help in advance.