MASTER_PORT seems to be mandatory for CHANGE MASTER for now, and not being set to 3306 by default.
Not having MASTER_PORT=3306 in an otherwise valid CHANGE MASTER command leads to wrong (generic?) error message:
ERROR 1200 (HY000): Misconfigured slave: MASTER_HOST was not set; Fix in config file or with CHANGE MASTER TO
even when MASTER_HOST is actually correctly given.