As the mistake was done via the god Indra, he grants Hanuman a desire that his human body could be as strong as Indra's Vajra, Which his Vajra may also not hurt him. Coupled with Indra other gods have also granted him needs: the God Agni granted Hanuman a want https://manueldtjyo.sharebyblog.com/36799122/hanuman-for-dummies