interbase中的存储过程问题?请指教?

[hjp0214]

我的一个存储过程如下:
CREATE PROCEDURE GETNEWID
RETURNS
(
NEWID integer
)
AS
declare variable nextid integer;
BEGIN
newid=0;
nextid=0;
for select max(ID) from client
order by id into :nextid
do
begin
if (nextid>0) then
begin
newid=newid+1;
exit;
end
else
newid=nextid;
end

END
表中都有数据,
而在intebase的IBCOLoNE执行"select * from getnewid"时却得到NUll,为什么呢?