As The mistake was accomplished via the god Indra, he grants Hanuman a desire that his human body might be as potent as Indra's Vajra, and that his Vajra also can not hurt him. In conjunction with Indra other gods have also granted him wishes: the God Agni granted Hanuman https://www.evernote.com/shard/s426/sh/92ffeab7-ae88-cc23-10fc-1daa8cd4e113/2bkAFXVOTFTdxbKxzLxQJl8B6AJV0IhNVOENCBdj9wqBWkWhYzI_gChrvQ