help!! with Secuence field (autonumeric)

[axel lope]

i try to create an autonumeric field, with select max for the pk field
but when y try to insert in this table return this error
ERROR 1100 (HY000): Table 'ALM_PROVEEDORES' was not locked with LOCK TABLES

what is the problem?

thanks....

CREATE TRIGGER TRG_PROV_BINSERT
BEFORE INSERT ON ALM_PROVEEDORES
FOR EACH ROW
BEGIN
DECLARE N INT;
SELECT ifnull(max(PRO_CORReLATIVO),0)+1 INTO N FROM
almacen.ALM_PROVEEDORES LIMIT 1;

SET NEW.PRO_CORRELATIVO = N;

END;