User blog comment:IamJakuhoRaikoben/Chapter 518: Master Enchant, Review./@comment-125.212.122.49-20170121132827/@comment-7418318-20170121194521

If it had been a simple fire magic spell (which was never stated by the way) and nothing else, it wouldn't have done any damage to Natsu, because Natsu is completely and utterly immune to normal and regular magical fire.

Even when his magic was completely shut down in Edolas, he could still put a torch in his mouth without getting hurt, so it's not just a magical immunity, but a physical one.

The only fire we've ever seen hurting him were the God Slayer flames and whatever bullshit Loke used in the ECS arc.