[8.1.4] Create index on timestamp fails

Hi all,

Using postgresql 8.1.4....

I have a table with an column:
entry_date | timestamp with time zone| not null

And when I try to create an index on it like so:
create index entry_date_idx on user_tracking(date_part('year',entry_date));

I get a
ERROR: functions in index expression must be marked IMMUTABLE

According to the mailing lists, this has been working since 7.4. What am I doing wrong?

tia,
arturo