Anyone found upgrade kits that work with heavy mithril items?

I crafted a set of heavy mithril stuff with blacksmithing, but i am unable to upgrade it with either Mithril, thorium, steel or iron upgrade kits... What gives?

It gives the error message that it cannot be ugpraded with that set, not the message that the item cannot be upgraded. Tried searching ah if there were different recipes for other upgrade sets i didnt know..

Did anyone find out?