MySQL: error 150 using ON UPDATE SET NULL and ON DELETE SET NULL, why?

i have this:

DROP TABLE IF EXISTS `sf_guard_user`;


CREATE TABLE `sf_guard_user`
(
 开发者_Go百科   `id` INTEGER(11)  NOT NULL AUTO_INCREMENT,
    `username` VARCHAR(128)  NOT NULL,
    PRIMARY KEY (`id`),
    UNIQUE KEY `sf_guard_user_U_1` (`username`)
)Type=InnoDB;


DROP TABLE IF EXISTS `shop_orders`;

CREATE TABLE `shop_orders`
(
    `orders_id` INTEGER(11)  NOT NULL AUTO_INCREMENT,
    `sfgu_id` INTEGER(11)  NOT NULL,

    PRIMARY KEY (`orders_id`),
    INDEX `shop_orders_FI_1` (`sfgu_id`),
    CONSTRAINT `shop_orders_FK_1`
        FOREIGN KEY (`sfgu_id`)
        REFERENCES `sf_guard_user` (`id`)
        ON UPDATE SET NULL
        ON DELETE SET NULL,

)Type=InnoDB;

and I'm getting this error:

1005 - Can't create table 'prueba1.shop_orders' (errno: 150)

if i do not remove the lines ON UPDATE SET NULL and ON DELETE SET NULL.

Any idea why?

Regards

Javi

I think that is because you declared the field NOT NULL