Every few minutes, the prompt "You have unspent skill points" pops up, even though I have no unspent skill points. When I press U I see my 4 skill trees, and they all have
Points: 0 below them.
Is there some other kind of skill point that can be spent that I don't know about, or is this a bug? I couldn't find anything by Googling the issue.
I'm running the latest version of Dying Light from Steam.