weird sum() results

I have a table with a filed named 'amount' and all the values are
positive values, but if i do this:

    select sum(amount) from table where state = 'CA';

I get a negative number.  Doing a

    select name, amount from table where state = 'CA' and amount < '0';

Returns 0 rows.  Any ideas as to why i'm getting a negative value for
the sum()?