As the mistake was carried out from the god Indra, he grants Hanuman a want that his body would be as robust as Indra's Vajra, Which his Vajra can also not harm him. Along with Indra other gods have also granted him needs: the God Agni granted Hanuman a desire https://share.evernote.com/note/72b9b1d6-5589-c620-5329-60af4177714f