Trying to create a table with a Foreign Key and getting the above error.

The first table creates fine:
Code:
CREATE TABLE ORDERS 
 (
    ORDERS_NUM         NUMBER       NOT NULL,
    CUST_NUM           NUMBER       NOT NULL,
    SHIP_STREET        VARCHAR(20)  NOT NULL,
    SHIP_CITY          VARCHAR(20)  NOT NULL,
    SHIP_STATE         VARCHAR(2)   NOT NULL,
    SHIP_ZIP           VARCHAR(5)   NOT NULL,
    SHIP_METHOD        VARCHAR(10)          ,
    ORDER_DATE         DATE         NOT NULL,
    SHIP_DATE          DATE                 ,
    PRIMARY KEY   (ORDERS_NUM, CUST_NUM),
    FOREIGN KEY   (CUST_NUM) REFERENCES CUSTOMER(CUST_NUM)
 );
The second table is where I get the error:
Code:
CREATE TABLE ORDERDETAIL
(
    DETAIL_NUM         NUMBER       NOT NULL,
    ORDERS_NUM         NUMBER       NOT NULL,
    PROD_NUM           NUMBER       NOT NULL,
    PROD_PRICE         DEC(10,2)            ,
    PROD_SHIP_COST     DEC(10,2)            ,
    QTY_PURCH          DEC(10,2)    NOT NULL,
    TOT_ORDER_WGHT     DEC(10,2)            ,
    ORDER_STAT	       DEC(10,2)    NOT NULL,
    TOT_PROD_COST      DEC(10,2)            ,
    TAX_RATE_ST        DEC(10,2)            ,
    ORDER_TAX          DEC(10,2)            ,
    TOT_SHIP_COST      DEC(10,2)            ,
    TOT_ORD_COST       DEC(10,2)            ,
    PRIMARY KEY   (DETAIL_NUM),
    FOREIGN KEY   (ORDERS_NUM) REFERENCES ORDERS(ORDERS_NUM),
    FOREIGN KEY   (PROD_NUM) REFERENCES PRODUCT(PROD_NUM)
);
Error is on this line:
Code:
    FOREIGN KEY   (ORDERS_NUM) REFERENCES ORDERS(ORDERS_NUM),
                                                 *
ERROR at line 17:
ORA-02270: no matching unique or primary key for this column-list
New to SQL, Can someone please assist? I am using Oracle SQL Plus.