I trying a this SQL:

select SESASU_ASUNTO as ASUNTO
from SESASU A join SESGRU G on G.SESGRU_CODIGO=A.SESASU_GRUPO
where G.SESGRU_MOSTRAR=1 and A.SESASU_SESION=1049
union
select F_STRBLOB(G.SESGRU_DESCRIP) as ASUNTO
from SESASU A join SESGRU G on G.SESGRU_CODIGO=A.SESASU_GRUPO
where G.SESGRU_MOSTRAR=0 and A.SESASU_SESION=1049
group by G.SESGRU_CODIGO

where SESASU_ASUNTO is a blob field and SESGRU_DESCRIP is a varchar.
I obtain this error message:
Invalid expression in the select list (not contained in either an
aggregate function or the GOUP BY clause).
If supress group by clause it's fine.
Why?