Not sure if this is intentional, but I am unable to put positive charms on crowns henchmen the turn I summon them. So far I've tried fire and elemental blades, but the game won't even let you select your henchman as a target. It might be because I always summon them on the first turn of battle, but maybe not. David Dreamcatcher, level 56
It's been that way at least since I bought my first henchman (I don't know how it was before that). You won't even be able to see what spell your henchman is about to cast on the first round they're in the battle.