Getting an 

> 
> Error Code: 1822. Failed to add the foreign key constraint. Missing
>  index for constraint 'subject\_ibfk\_1' in the referenced table
>  'enrolment'
> 
> 
> 

when attempting to create the `subject` table. The problem is, the error does not occur on the previous table `student`. The data types are the same, and the primary keys are defined.

This error occurs for both the `enrolment` and `grade` tables.

```
create table enrolment(
    stud_id char(9) not null,
    subj_code char(8) not null,
    semester tinyint unsigned not null,
    year smallint unsigned not null,
    comment text,

    primary key (stud_id, subj_code, semester, year)
);

create table grade(
    stud_id char(9) not null,
    subj_code char(8) not null,
    semester tinyint unsigned not null,
    year smallint unsigned not null,
    grade tinyint unsigned,

    primary key (stud_id, subj_code, semester, year)
);

create table student(
    stud_id char(9) not null,
    stud_name char(30),
    stud_phone char(12),
    stud_date_of_birth date,
    stud_city char(26),
    stud_address char(30),
    stud_postcode char(4),

    primary key (stud_id),

    foreign key (stud_id)
        references grade(stud_id),
    foreign key (stud_id)
        references enrolment(stud_id)
);

create table subject(
    subj_code char(8) not null,
    subj_title char(40),

    primary key (subj_code),

    foreign key (subj_code)
        references enrolment(subj_code),

    foreign key (subj_code)
        references grade(subj_code)
);

```