As the mistake was done because of the god Indra, he grants Hanuman a want that his overall body will be as robust as Indra's Vajra, Which his Vajra could also not harm him. Along with Indra other gods have also granted him needs: the God Agni granted Hanuman a https://baglamukhimalamantra50504.tinyblogging.com/the-basic-principles-of-hanuman-80796840