error can't select an agregate function

Hello,

first sorry for my english but it`s not my natural language.

I've a question. These Days if made an update from postgresql 7.0 to 7.1 and
since this time I was unable to enter SQL-Queries like :
select sum(offline-online) from online;
where offline and online are attributes of time

The reported errormessage is like : error can't select from aggregate function.

Is it generally impossible since this version or do I have to configure
something.

I think this question is already discussed but please if someone find it useful
to tell me how often this question is allready postet please send me what I do
have to do to fix the problem and I'll never post a question again, because
everything else is working fine.

Regards Jörg