表达式不能用作分配目标

我有一个具有以下查询的函数，但它返回上述错误.我认为问题在于aty的粗体字

I have a function with following query but it returns above error. I think the problem is with bold line specifically with aty

CREATE OR REPLACE FUNCTION QtyStkToQty ( itemcode VARCHAR2, qty NUMBER, ConUOM VARCHAR2 ) RETURN VARCHAR IS vSupuomcon float;

 vConuomcon float;

 vuom varchar2(3);

 vunits number;

 retval float;

 nqty number;


CURSOR cur_uom_units(c_itemcode varchar2) IS
SELECT a.iu_uom_code,
       a.iu_units
FROM
  (SELECT iu_uom_code,
          iu_units
   FROM sm_uom
   WHERE iu_item_code=c_itemcode
     AND iu_freez='N'
     AND rownum <= 3
   ORDER BY iu_units ASC) a
ORDER BY iu_units DESC;

 BEGIN retval:= 0;

 IF qty<> 0 THEN OPEN cur_uom_units(itemcode);

 FETCH cur_uom_units INTO vuom,
                          vunits;

 LOOP FETCH cur_uom_units INTO vuom,
                               vunits;

 EXIT WHEN cur_uom_units% NOTFOUND;

 retval := retval +' '+ CAST(FLOOR(qty/ vunits) AS varchar2);

 qty := qty-((qty/ vunits)*vunits);

 END LOOP;

 CLOSE cur_uom_units;

 ELSE retval := 0;

 RETURN retval;

 END IF;

 END;