Cannot add foreign key constraint 2

im try create a foreign key to Poblacion, on table cp_cliente, but i cant, i take the error "cannot add foreign key", im try change the name, but i cant. If i delete Poblacion foreign key the script work fine.

CREATE TABLE provincias (

Cod_provincia INT(2) PRIMARY KEY,

Provincia VARCHAR(50) NOT NULL );

CREATE TABLE cp_cliente (

CP CHAR(5),

Cod_provincia INT(2),

Poblacion VARCHAR(70),

PRIMARY KEY (CP, Poblacion, Cod_provincia),

FOREIGN KEY (Cod_provincia) REFERENCES provincias(Cod_provincia) ON DELETE CASCADE ON UPDATE CASCADE );

CREATE TABLE cliente (

DNI CHAR(9) PRIMARY KEY,

Nombre VARCHAR(20) NOT NULL,

Apellidos VARCHAR(20) NOT NULL,

Direccion VARCHAR(50) NOT NULL,

CP CHAR(5),

Cod_provincia INT(2),

Poblacion VARCHAR(70),

FOREIGN KEY (CP) REFERENCES cp_cliente(CP) ON DELETE SET NULL ON UPDATE CASCADE,

FOREIGN KEY (Cod_provincia) REFERENCES cp_cliente(Cod_provincia) ON DELETE SET NULL ON UPDATE CASCADE,

FOREIGN KEY (Poblacion) REFERENCES cp_cliente(Poblacion) ON DELETE SET NULL ON UPDATE CASCADE );

What is the problem? I can't see him :(

Sorry my bad english.