我这个语句错在哪里了呀

我这个语句错在哪里了呀
建表语句是这个
CREATE TABLE customers (
customer_id INTEGER CONSTRAINT customers_pk PRIMARY KEY,
first_name VARCHAR2(10) NOT NULL,
last_name VARCHAR2(10) NOT NULL,
dob DATE,
phone VARCHAR2(12)
);
我插入数据是这个
INSERT INTO customers (
customer_id,first_name,last_name,dob,phone
) VALUES (
1,'John','Brown','01-JAN-1965','800-555-1211'
);
INSERT INTO customers (
customer_id,first_name,last_name,dob,phone
) VALUES (
2,'Cynthia','Green','05-FEB-1968','800-555-1212'
);
INSERT INTO customers (
customer_id,first_name,last_name,dob,phone
) VALUES (
3,'Steve','White','16-MAR-1971','800-555-1213'
);
INSERT INTO customers (
customer_id,first_name,last_name,dob,phone
) VALUES (
4,'Gail','Black',NULL,'800-555-1214'
);
INSERT INTO customers (
customer_id,first_name,last_name,dob,phone
) VALUES (
5,'Doreen','Blue','20-MAY-1970',NULL
);
然后就提示我说无效的月份?咋回事呢,咋修改呢