Mysql 5.6 doesn't want to set a column to NULL - php

Question

I am about 98% sure this query has been working properly on mysql 5.1. We have upgraded one of our machines to mysql 5.6 and I am running an update and its getting a database error with this query:
UPDATE diagnostic
LEFT JOIN contact ON diagnosticdata_suppliercontact = contact_id
SET diagnosticdata_suppliercontact = NULL
WHERE (!contactdata_issupplier) OR (contact.contact_id IS NULL);
However the table schema allows it to be NULL. Does anyone know of any problems or changes that happened in mysql 5.6 that could cause us grief?
Here is a description of the table:
mysql> describe diagnostic;
+--------------------------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------------------------+-------------+------+-----+---------+----------------+
| diagnostic_id | int(11) | NO | PRI | NULL | auto_increment |
| diagnostic_time | int(15) | NO | | NULL | |
| diagnostic_user | int(15) | NO | | NULL | |
| diagnosticdata_time | int(15) | NO | | NULL | |
| diagnosticdata_user | int(15) | NO | | NULL | |
| diagnosticdata_name | varchar(50) | NO | | NULL | |
| diagnosticdata_suppliercontact | int(11) | YES | MUL | NULL | |
+--------------------------------+-------------+------+-----+---------+----------------+
7 rows in set (0.00 sec)
ERROR 1048 (23000): Column 'diagnosticdata_suppliercontact' cannot be null
CREATE STATEMENT IS HERE TO there are triggers:
delimiter $$
CREATE TABLE `diagnostic` (
`diagnostic_id` int(11) NOT NULL AUTO_INCREMENT,
`diagnostic_time` int(15) NOT NULL,
`diagnostic_user` int(15) NOT NULL,
`diagnosticdata_time` int(15) NOT NULL,
`diagnosticdata_user` int(15) NOT NULL,
`diagnosticdata_name` varchar(50) NOT NULL,
`diagnosticdata_suppliercontact` int(11) DEFAULT NULL,
PRIMARY KEY (`diagnostic_id`),
KEY `diagnostic_suppliercontact` (`diagnosticdata_suppliercontact`) USING BTREE,
CONSTRAINT `fk_diagnosticdata_suppliercontact_contact` FOREIGN KEY (`diagnosticdata_suppliercontact`) REFERENCES `contact` (`contact_id`)
) ENGINE=InnoDB AUTO_INCREMENT=188 DEFAULT CHARSET=utf8$$
CREATE
DEFINER=`dotuser`#`localhost`
TRIGGER `ezymerged`.`diagnostic_insert`
AFTER INSERT ON `ezymerged`.`diagnostic`
FOR EACH ROW
BEGIN
REPLACE INTO ezymerged_history.diagnostic
SELECT diagnostic.* FROM ezymerged.diagnostic
WHERE diagnostic_id=NEW.diagnostic_id;
END
$$
CREATE
DEFINER=`dotuser`#`localhost`
TRIGGER `ezymerged`.`diagnostic_update`
AFTER UPDATE ON `ezymerged`.`diagnostic`
FOR EACH ROW
BEGIN
REPLACE INTO ezymerged_history.diagnostic
SELECT diagnostic.* FROM ezymerged.diagnostic
WHERE diagnostic_id=NEW.diagnostic_id;
END
$$

Answer 1

While the diagnosticdata_suppliercontact allows for NULLs the constraint:
CONSTRAINT `fk_diagnosticdata_suppliercontact_contact`
FOREIGN KEY (`diagnosticdata_suppliercontact`)
REFERENCES `contact` (`contact_id`)
will take precedence, and requires the value in diagnosticdata_suppliercontact to have a matching value in the contact_id field in the contact table.