我这个语句错在哪里了呀

建表语句是这个

CREATETABLEcustomers(

customer_idINTEGERCONSTRAINTcustomers_pkPRIMARYKEY,

first_nameVARCHAR2(10)NOTNULL,

last_nameVARCHAR2(10)NOTNULL,

dobDATE,

phoneVARCHAR2(12)

);

我插入数据是这个

INSERTINTOcustomers(

customer_id,first_name,last_name,dob,phone

)VALUES(

1,'John','Brown','01-JAN-1965','800-555-1211'

);

INSERTINTOcustomers(

customer_id,first_name,last_name,dob,phone

)VALUES(

2,'Cynthia','Green','05-FEB-1968','800-555-1212'

);

INSERTINTOcustomers(

customer_id,first_name,last_name,dob,phone

)VALUES(

3,'Steve','White','16-MAR-1971','800-555-1213'

);

INSERTINTOcustomers(

customer_id,first_name,last_name,dob,phone

)VALUES(

4,'Gail','Black',NULL,'800-555-1214'

);

INSERTINTOcustomers(

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