Can't declare co-belligerent?

[Morik]

I just started an Otto game. I declared on Aq Qoyunlu, who was allied with Karaman. I was unable to tick the co-belligerent box, and now I'm left with a 15 year peace treaty with Karaman (they had 4 provinces, and at double warscore cost I was unable to annex them... had to take 3 provinces and leave their capital).

What decides whether I can name someone as a co-belligerent?

 

[Alaitoc]

You can't declare them co-belligerent if you already have a truce with them, in my experience. Would be nice if the tooltip told you that though.