SP2-0552: Bind variable "B1" not declared. How do I do it?

Hi,
Im trying to run a SQL in SQLPLUS (11.2.0.2)

SQL> variable sku number;
SQL> exec :sku :=1757933

PL/SQL procedure successfully completed.


SELECT nvl (AVG (ship_carton_wt), 0)
  FROM item_supp_country
 WHERE item IN (
          SELECT sku
            FROM desc_look
           WHERE (CLASS, dept, subclass) = (SELECT CLASS, dept, subclass
                                              FROM desc_look
                                             WHERE sku = :b1 AND ROWNUM = 1)
             AND system_ind = 'F')

SP2-0552: Bind variable "B1" not declared.

How Do I declare B1?

Regards
U