Altar of Corruption won't apply to Skill Gems

I'm now on my second character of the league, and have been unable to try and super-corrupt any skill gems at the Altar in the Locus of Corruption. I've tried several different spell gems, and always get the system message- "Failed to apply item: Item cannot be corrupted." The gems are not already corrupted. Would really like to try and get some lvl 21 Vaal gems this league... Thanks for any assistance.