[SQL] Subselect has too many or too few fields

I have a problem with query I thought must works:

c_flats=> select source, max(up_date) as max_update, count(*) from flats where u
up_date in (select max(up_date) from flats group by source) group by source;
ERROR: parser: Subselect has too many or too few fields.

Internal subselect produces as expected:
c_flats=> select max(up_date) from flats group by source;
max
----------
15-03-1999
11-03-1999
24-03-1999
24-03-1999
24-03-1999

And I don't see a reason my query doesn't works.
If I substite the result of internal subselect I get what I need (there are
cyrillic characters in <source>)

c_flats=> select source, max(up_date) as max_update, count(*) from flats where u
p_date in ('15-03-1999','11-03-1999','24-03-1999','24-03-1999','24-03-1999') gro
up by source;
source |max_update|count
--------------------------+----------+-----
ИНКОМ 369-00-25, 365-10-38|15-03-1999| 674
Корнет 912-00-12 |11-03-1999| 47
МИАН 974-62-62 |24-03-1999| 393
МИЭЛЬ 217-30-20, 217-39-13|24-03-1999| 1
МЦБН <null> |24-03-1999| 141
(5 rows)

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83