Cannot add foreign key here? - mysql

Question

So I'm trying to create tables and I can't for the life of me understand why i keep getting and error saying "Cannot add foreign key to constraint".
The types are the same, the parent is a primary key, and they're NULLness is the same.
The problem is in the line in the create table for CDSingers where it says:
foreign key (track_num) references CDTracks (track_num),
(it's near the end)
It's the only table that won't be created and it's because of this line.
Please help.
(some of the other tables have been ommitted since they aren't connected)
create table CD
(
num int NOT NULL,
producer varchar(100) NOT NULL,
cd_number varchar(100) NOT NULL,
title varchar(100) NOT NULL,
type varchar(100) ,
band_name varchar(100) ,
production_date DATE NOT NULL,
price double CHECK (price >= 0),
foreign key (type) references MusicType (type),
foreign key (band_name) references Band (band_name),
primary key (num),
unique (producer, cd_number)
);
create table CDTracks
(
num int NOT NULL,
track_num int NOT NULL,
song_name varchar(100) NOT NULL,
minute int NOT NULL,
foreign key (num) references CD (num),
primary key (num, track_num)
);
create table Singer
(
id int NOT NULL,
singer_firstname varchar(100) NOT NULL,
singer_lastname varchar(100) NOT NULL,
primary key (id)
);
create table CDSingers
(
num int NOT NULL,
track_num int NOT NULL,
singer_id int NOT NULL,
foreign key (num) references CDTracks (num),
foreign key (track_num) references CDTracks (track_num),
foreign key (singer_id) references Singer (id),
primary key (num, track_num, singer_id)
);

Answer 1

You declare composite foreign keys like this:
foreign key (num, track_num) references CDTracks (num, track_num),