As the error was carried out from the god Indra, he grants Hanuman a want that his entire body might be as robust as Indra's Vajra, Which his Vajra also can not harm him. In addition to Indra other gods have also granted him wishes: the God Agni granted Hanuman https://www.diigo.com/item/note/avyq4/182s?k=a383985d376117bfd69311ed4c19ce87