"Come to London and shoot a few shots for 'Ra.One' for luck, " Shah Rukh posted to Karan on micro-blogging site Twitter.
Karan, who directed his films with Shah Rukh in the lead, replied: "Will arrive for sure bhai (brother)... though my action and sci-fi skills are highly questionable!"
In fact, Karan is producing "Agneepath" remake but he will not wield the megaphone for the movie as he doubts his capability of directing an action film.