Airborne upgrade

[unmerged(47302)]

1.04 GC36 US
In the production list, I have a serial production of 1*3 airborne inf lvl 1
I have not researched yet Airborne lvl2 and the icone propose me the "+" to upgrade production.
when I click on it, nothing happens, which is OK but "+" should not appear.

 

[aasand]

I bet you forget the brigade you have attached is also upgradable..