1215 Cannot add foreign key constraint

CREATE DATABASE java1707;
USE java1707;
CREATE TABLE tab(
tab_id VARCHAR(10),
location VARCHAR(50));
CREATE TABLE student(
stu_id VARCHAR(10)PRIMARY KEY,
stu_name VARCHAR(10),
tab_id VARCHAR(10),
age INT
);
ALTER TABLE student ADD FOREIGN KEY(tab_id) REFERENCES tab(tab_id);

http://blog.csdn.net/yiwangxiblog/article/details/52269527