A question about GROUP BY (ORA-00979: not a GROUP BY expression)

Hi, 
I'm trying the same query in Oracle 9i and Oracle 817 and receive
different results. Can you help to explain the reason? Thanks!

The query is
select cast(deptno as number(10)), min(salary) 
from table1
group by cast(deptno as number(10));

The types of the columns are
 DEPTNO                                   NUMBER(3)
 SALARY                                   NUMBER(10)

In Oracle 9i, I received the following error
ERROR at line 1:
ORA-00979: not a GROUP BY expression

In Oracle 817, correct result is returned. 

Is there any new restrictions introduced in Oracle 9i? What is the
reason Oracle 9i raises the error?

Thanks,
Lan