problem with partitioned table and indexed json field

Hi,

I have a partitioned table events, with one partition for each month, eg
events_2013_03. The partition is done on the field timestamp, and
constraints are set, but insertion of data is done in the partition
directly (so not with a trigger on the events table)
The field event is of type json, and has a field '_id', which I can access:

=> select event->>'_id' from events limit 1;
?column?
--------------------------
4f9a786f44650105b50aafc9

I created an index on each partition of the table, but not on the events
table itself:
create index events_${y}_${m}_event_id_index on events_${y}_${m}
((event->>'_id'));

Querying the max event_id from a partition works fine:
=> select max(event->>'_id') from events_2013_03;
max
--------------------------
5158cdfe4465012cff522b74

However, requesting on the parent table does return the whole json field,
and not only the '_id':
=> select max(event->>'_id') from events;
{"_id":"526eb3ad4465013e3e131a43","origin":..... }

An explain returns an error:
=> explain select max(event->>'_id') from events;
ERROR: no tlist entry for key 2

This problem appeared when I created the indexes, and removing the index
make the explain work fine, but the plan implies a sequential scan on the
tables which is exactly what I wanted to avoid with the indexes.

Does someone have an explanation, and possibly a way to solve this problem?

thanks

Raph