Slotting a vitality support gem in my Demon Form makes it uncastable no matter the amount of spirit.

Whenever I slot the vitality support gem in the Demon Form skill it makes it uncastable no matter what. I have tried taking off all other spirit summons and I still cant use it. Once I remove the gem and put in on something else I can case Demon Form. This is the most needed support gem and I can't use it :(

What about tha support gems requirements? Demon form removes your weapons, so maybe you are missing stat for that support gem cause of that?