to_char not IMMUTABLE?

I had a problem when upgrading a database from 8.1.4 to 8.2.1:
Sorry, the error messages are in german.

pg_restore: [archiver (db)] Error while PROCESSING TOC:
pg_restore: [archiver (db)] Error from TOC entry 1531; 1259 3477393 INDEX idx_inspektionen_dat_inspektion
pg_restore: [archiver (db)] could not execute query: FEHLER: Funktionen im Indexausdruck muessen als IMMUTABLE markiert sein
Command was: CREATE INDEX idx_inspektionen_dat_inspektion ON inspektionen USING btree (to_char(dat_inspektion, 'yyyy'::text));
WARNING: errors ignored on restore: 1

to_char(timestamp, 'yyyy') should be constant and marked immutable, or am I wrong here? Or is it not marked immutable because of possible changes on date_format?

Regards,
Mario Weilguni