Xml parsing in iPhone

Question

Hello I have created some registration process in iPhone but when i am sending data with using XML parsing that time i get an error that is:- * Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '* -[NSCFString stringByReplacingOccurrencesOfString:withString:options:range:]: nil argument'

so help me out to solve this error

Thanks Kunal