"Show on selected" acts as "hide on selected"

The weirdest thing is happening to me right now.

I want a widget to appear on a specific post, so I set it for “show on selected” and target the url of the post I want it to appear on.

What it does, is that it actually hides it from this specific post and shows it everywhere else.

I’ve tried all the techniques related to show/hide on selected problems I’ve read on this forum, using */postname and postname (without other characters) but nothing works.

Is it actually possible to get a widget to work on a specific post, or does this only works for pages?

http://wordpress.org/plugins/widget-context/