can't add foreign key in mysql?

create table department
    (dept_name        varchar(20), 
     building        varchar(15), 
     budget                numeric(12,2) check (budget > 0),
     primary key (dept_name)
    );

create table course
    (course_id        varchar(8), 
     title            varchar(50), 
     dept_name        varchar(20),
     credits        numeric(2,0) check (credits > 0),
     primary key (course_id),
     foreign key (dept_name) references department
        on delete set null
    );

显示出错can't add foreign key 。
版本：MySQL5.6
但在MySQL5.1上能运行

在网上各种找原因，都搞不定。最后是这样搞定的，太坑了：

 1 create table department
 2     (dept_name        varchar(20), 
 3      building        varchar(15), 
 4      budget                numeric(12,2) check (budget > 0),
 5      primary key (dept_name)
 6     );
 7 
 8 create table course
 9     (course_id        varchar(8), 
10      title            varchar(50), 
11      dept_name        varchar(20),
12      credits        numeric(2,0) check (credits > 0),
13      primary key (course_id),
14      foreign key (dept_name) references department（dept_name）
15         on delete set null
16     );