Fire Spell on Hit and Volcano

I cannot slot Volcano into Smith of Kitava's skill Fire Spell on Hit. It says fire spell in the description and Volcano is a fire spell. I know it has a channeling component, which would not work (although this also needs to be stated clearly), but the Volcano can be used without channeling it as well. so I expect to be able to socket it in the skill.

Yup, same. If it's intended then it's not explained anywhere.