Hysseg's Claw Skill "Cackling Companions" unable to equip support gems

I have a Hysseg's Claw with Level 12 Cackling Companions that works fine. It has three gem slots.

I bought an upgrade with Level 17 Cackling Companions and it has four gem slots, but for some reason it is unable to equip support gems. I'm not sure this is for all support gems, but if I use the same support gems I was already using for the level 12 skill, it says "This support can't be socketed into this skill."