Can't socket supports into Herald of Thunder

I just used a Lesser Jeweler's Orb on Herald of Thunder to get a third support gem slot, however ALL of the support gems say "This support can't be socketed into this skill".

Even if I uncheck "show recommended gems" and look at every single support gem in the entire game, they are all grayed out with the same message.

If I try to take a gem that's already cut and socket it into Herald of Thunder, the game won't allow me to socket the support. Says the same error.

What gives?

*Edit* - Figured it out. One of the other supports linked to Herald was Execute, which required more strength than I currently had after swapping a piece of gear. Apparently this prevented me from socketing additional gems into the skill, even if they were INT / DEX gems. Would be helpful if the error provided was less vague, or if support gems automatically unequipped if stat requirements weren't met.