CASE

Hi everyone, I'm trying to figure out how to get CASE working in a sum
aggregate function.  Pretty simple query:

select sum(numbers) from lists where uid=1;

It's possible that there may be 0 rows where uid == 1.  Instead of
returning 1 blank row, I want it to return '0'.  So, I tried this:

select case(sum(numbers)) when null then 0 else sum(numbers) end from
list_results;

That... Doesn't work.  Any suggestions on how to do this and where I can
find some decent information on use of CASE?