Ruzhan, The Blazing Sword - Unable to add Spell Cascade

In the gemcutting UI For Ruzhan, The Blazing Sword we are able to cut Spell Cascade gem for the new Sorceress ascendancy but unable to equip it. It says Warning: This support can't be socketed into this skill.

It would be awesome if it was fixed so we can throw more fire traps per cast.