给从表设置了外键级联更新和删除置空不起效果

MariaDB [webshopping]> alter table commoditys add CONSTRAINT commoditys_commodityclasses_jbfk_1 foreign key(commodityClassId) references commodityclasses(commodityClassId) on update cascade on delete set null;
Query OK, 19 rows affected (0.052 sec)
Records: 19 Duplicates: 0 Warnings: 0
MariaDB [webshopping]> delete from commodityclasses where commodityClassId > 5;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (webshopping.commoditys, CONSTRAINT FK4616CEE64542CEEB FOREIGN KEY (commodityClassId) REFERENCES commodityclasses (commodityClassId))
如上，为什么给从表设置了外键级联更新和删除置空不起效果？