PL / SQL触发器返回缺货错误：使用编译错误创建的触发器

我有以下表格：

order_lines table
    Name  Null?  Type  
    ORDER_ID  NOT NULL  NUMBER(5)  
    PRODUCT_ID  NOT NULL  NUMBER(4)  
    ACTUAL_PRICE     NUMBER  
    DISCOUNT     NUMBER  
    QUANTITY     NUMBER  
    TOTAL_AMOUNT     NUMBER  

和

Product_inventory table
Name  Null?  Type  
PRODUCT_ID  NOT NULL  NUMBER(4)  
QTY_ON_HAND     NUMBER  
QTY_ON_ORDER     NUMBER  
DATE_ORDERED     DATE  
DELIVERY_DATE     DATE 

我正在尝试使用一个触发器来检查订单数量，看看是否有库存，并显示一条消息。我当前的触发器正在编译，但有一些错误：

    CREATE OR REPLACE TRIGGER check_order_line  
  BEFORE INSERT OR UPDATE ON order_lines
  for each row
DECLARE
  l_current_stock product_inventory.qty_on_hand%type;
BEGIN 
  select product_inventory.qty_on_hand
    into l_current_stock
    from product_inventory, order_lines
   where product_inventory.product_id = :new.product_id;
  if(:new.quantity > l_current_stock) then
    RAISE_APPLICATION_ERROR(-20103, 'Insufficient Stock');
  else
    update product_inventory
   set qty_on_hand = qty_on_hand - :new.quantity
 where product_inventory.product_id = :new.product_id;
  end if;
END;  

在测试触发器时，出现以下错误：

 insert into order_lines values (388,1023,100,20,2,160)
            *

ERROR at line 1: 
ORA-01422: exact fetch returns more than requested number of rows 
ORA-06512: at "DBA643.CHECK_ORDER_LINE", line 4 
ORA-04088: error during execution of trigger 'DBA643.CHECK_ORDER_LINE' 

我为什么得到这个？