Resolved Can't upgrade my Plasma Assault Rifle at an Weapon Upgrade Anvil

Hello, I was happy to find my Plasma Assault Rifle, it is probably one of my favorite weapons now, but I noticed you cant upgrade it, I tried if thats the problem with all the 1.3 weapons, but I found that you can upgrade the Ray Gun and the Shock Baton. (not counting the Drill Spear, its not a unique weapon) So I seriously dont know whats the problem here, pls fix?

 

The Plasma Assault Rifle is most likely tier 6 by default, and so it can't be upgraded any further.

 

Thanks, I kind of didnt even think about the fact that it upgrades them to T6 and because of that T6 weps cant be upgraded, well thanks!