ERROR 1005 (HY000): Can't create table 'Azienda.#sql-697_c9' (errno: 150)

[andrea esposito]

Hi,all
I have this two tables:

mysql> show create table Dipartimento\G
*************************** 1. row ***************************
Table: Dipartimento
Create Table: CREATE TABLE `Dipartimento` (
`Nome` varchar(15) NOT NULL,
`Sede` varchar(15) NOT NULL,
`Telefono` varchar(13) NOT NULL,
PRIMARY KEY (`Nome`),
KEY `Sede` (`Sede`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Insieme di dipartimenti'
1 row in set (0,22 sec)

mysql> show create table Sede\G
*************************** 1. row ***************************
Table: Sede
Create Table: CREATE TABLE `Sede` (
`Citta` varchar(15) NOT NULL COMMENT 'Citt� sede di un dipartimento',
`Via` varchar(50) NOT NULL,
`NumeroCivico` varchar(4) NOT NULL,
`CAP` varchar(6) NOT NULL,
PRIMARY KEY (`Citta`),
KEY `Citta` (`Citta`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COMMENT='Citt� dove hanno sede i dipartimenti'
1 row in set (0,00 sec)

when I write the alter table command for adding a foreign key:

ALTER TABLE `Dipartimento` ADD FOREIGN KEY ( `Sede` ) REFERENCES `Azienda`.`Sede` (`Citta`) ON DELETE SET NULL ON UPDATE CASCADE ;

I get this error:
ERROR 1005 (HY000): Can't create table 'Azienda.#sql-697_c9' (errno: 150)
I think that this error is related to the attribute types but i can't "see" it.Any suggestion?Thank you all.