index refuses to build

Hi list,

I'm wrong somewhere, but where?:

CREATE INDEX tst1m_name_lu_ix ON tst1m(lower(unaccent(name)));
ERROR: functions in index expression must be marked IMMUTABLE

Decomposing it reveals that it is the 'unaccent' part that
blocks:

CREATE INDEX tst1m_name_lu_ix ON tst1m(unaccent(name));
ERROR: functions in index expression must be marked IMMUTABLE

From what I found on the web I also tried to cast it to text,
but the error's still here:(

JY
--
One of the signs of Napoleon's greatness is the fact that he once
had a publisher shot.
-- Siegfried Unseld