MySQL server 5.1 trigger problem

I have implemented a trigger. But once I try to update the table it is giving me the error: >can't update table 'booking' in stored function/trigger because it is already used by the statement which invoked this stored function/trigger This is the trigger DELIMITER // CREATE TRIGGER CANCEL_BOOK AFTER UPDATE on BOOKING FOR EACH ROW BEGIN DELETE FROM BOOKING WHERE NEW.F_ID=NULL AND NEW.H_ID=NULL; END // DELIMITER ; please help