setting a foreign key to an empty string I'm trying to set a foreign key to an empty string but i keep getting this error.

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (`msis230`.`deptchair`, CONSTRAINT `deptchair_ibfk_1` FOREIGN KEY (`PROFESSORID`) REFERENCES `professor` (`PROFID`))

this is my code:
INSERT INTO DEPTCHAIR (PROFESSORID, DEPTID)
VALUES ('SJ001', 'MSIS');
INSERT INTO DEPTCHAIR (PROFESSORID, DEPTID)
VALUES ('', 'BIO');
INSERT INTO DEPTCHAIR 
VALUES ('JS001', 'PHY');
INSERT INTO DEPTCHAIR 
VALUES  ('NS001', 'MKT');
INSERT INTO DEPTCHAIR 
VALUES ('', 'ECO');
There are two approaches you can take
APPROACH #1 : Add to professor table
INSERT INTO professor (PROFID) VALUES ('');

Then, all INSERTs to DEPTCHAIR will work.
APPROACH #2 : Disable Foreign Key Check
SET foreign_key_checks = 0;
INSERT INTO DEPTCHAIR (PROFESSORID, DEPTID)
VALUES ('', 'BIO');
SET foreign_key_checks = 1;