Issue with creation of Partial_indexes (Immutable?)

CREATE INDEX idx_d_trh_code_id_partial
  ON xmms.d_trh_table
  USING btree
  (code_id) where code_id not in ('P000','000') and code_id is not null;
ERROR:  functions in index predicate must be marked IMMUTABLE

Just trying something new.

I want to create partial indexes on code_id which are not null and not
P000/000

the ones I want are like HMD11 or UE935 or OIOR11 etc.

not sure where the IMMUTABLE part is coming from..

Clue?