外键约束错误

CREATE TABLE orders
(
o_num int (11) PRIMARY KEY   AUTO_INCREMENT ,
o_date date ,
c_id varchar (50) NOT NULL ,
CONSTRAINT FK_EMP_DEPT1 FOREIGN KEY (c_id) REFERENCES customers_info (c_num)
) ;
 创建提示外键约束添加错误，请帮我看下，错误提示
 

ERROR 1215 (HY000): Cannot add foreign key constraint
错误1215（HY000）：不能添加外键约束