As the mistake was done from the god Indra, he grants Hanuman a would like that his overall body will be as strong as Indra's Vajra, Which his Vajra could also not hurt him. In conjunction with Indra other gods have also granted him wishes: the God Agni granted Hanuman https://www.reddit.com/user/SwimKey4360/comments/1dfi4sn/embrace_serenity_and_culture_at_the_hanuman_mandir/