SQL in oracle help

hi, I have this simple code, is a procedure for oracle..

DECLARE
ab integer;
ac integer;
BEGIN
ab := 1;
ac := 3;
for i in ab..ac loop
if i=3 then
insert into zona(codigo_zona, numero_zona) values (4,4);
end if;
end loop;
END;

and oracle states that is INVALID, and doesn't show me anything in the error display box, so if anyone can help me I will apreciated it...

dani2003_oracle