I think the problem is somehow related to roman numeral conversion when
some values are null.
The following seems to work:
select case when gen is not null then to_char(gen,'rn') else null end, case when gen is not null then to_char(gen+1,'rn') else null end from cname;
While the following doesn't:
select to_char(gen,'rn'), to_char(gen+1,'rn') cname;