mysql procedure if condition

I have a mysql procedure that it doesn´t work an IF condition with AND. I'm not sure what is the correct syntax.

 CREATE PROCEDURE insere_orcamento(
 IN ida VARCHAR(10),
 IN volta VARCHAR(10),
 OUT result INT
)
BEGIN

 IF (ida='sim' AND volta=NULL) THEN
    SET result = 50;
 ELSEIF (ida=NULL AND volta='sim') THEN
    SET result = 51;
 END IF;

END