As the error was performed with the god Indra, he grants Hanuman a desire that his system could well be as solid as Indra's Vajra, Which his Vajra could also not harm him. Coupled with Indra other gods have also granted him wishes: the God Agni granted Hanuman a desire https://www.tumblr.com/hanumanmandiradelaide/742853809162829824/spiritual-experience-puja-at-hanuman-mandir