As the error was done from the god Indra, he grants Hanuman a want that his entire body could be as strong as Indra's Vajra, Which his Vajra also can not harm him. Together with Indra other gods have also granted him needs: the God Agni granted Hanuman a desire https://milokptuv.ampedpages.com/the-smart-trick-of-hanuman-that-no-one-is-discussing-63822283