title: MySQL: error 150 using ON UPDATE SET NULL and ON DELETE SET NULL, why?
body: <p>i have this:</p>
  
  <pre><code>DROP TABLE IF EXISTS `sf_guard_user`;
  
  
  CREATE TABLE `sf_guard_user`
  (
      `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;
  </code></pre>
  
  <p>and I'm getting this error:</p>
  
  <blockquote>
    <p>1005 - Can't create table
    'prueba1.shop_orders' (errno: 150)</p>
  </blockquote>
  
  <p>if i do not remove the lines ON UPDATE SET NULL and ON DELETE SET NULL.</p>
  
  <p>Any idea why?</p>
  
  <p>Regards</p>
  
  <p>Javi</p>