ERROR: loop variable of FOREACH must be a known variable or list of variables

I have few FOREACH loops in my functions. The loops seem to perfectly agree with the official postgresql documentation. This error message is returned:

ERROR: loop variable of FOREACH must be a known variable or list of variables
SQL state: 42601

Google found nothing for postgresql "loop variable of FOREACH". I solved this somehow for my older foreach loops, but I don't remember how, and I can't find any difference that could be significant for the solution of this error.

I have PostgreSQL 9.3.4 and pgAdmin III 1.18.1 on Windows XP 32 bit. Here are interresting parts of the functions:

CREATE OR REPLACE FUNCTION vloz_kont() RETURNS trigger AS $
DECLARE
  _idmat integer;
  _nazmat text;
  _zast text;
  sj_nad text;
  ...
BEGIN

...

--bugged loop
FOREACH _mat IN ARRAY NEW._material::text[] LOOP
  SELECT split_part( _mat, ' ', 1 ) INTO _nazmat;
  SELECT rtrim( ltrim(split_part( _mat, ' ', 2 ), '(' ), ')' ) INTO _zast;
  SELECT id INTO _idmat FROM pro_material WHERE popis = _nazmat;
  IF _idmat IS NOT NULL THEN
    INSERT INTO material_konst ( id_mat, kod_sj, kod_akce, zastoupeni )
    VALUES ( _idmat, NEW._kod_sj, NEW._kod_akce, _zast );
  ELSE
    INSERT INTO material_konst ( id_mat, kod_sj, kod_akce, jiny_nazev, zastoupeni )
    VALUES ( 8, NEW._kod_sj, NEW._kod_akce, _nazmat, _zast );
  END IF;
END LOOP;

...

--OK LOOP
FOREACH sj_nad IN ARRAY NEW._nad::text[] LOOP
  INSERT INTO s_vztah (nad, pod, kod_akce, typ_vztahu)
  VALUES(
    sj_nad,
    NEW._kod_sj,
    NEW._kod_akce,
    1
  );
END LOOP;

...

RETURN NEW;
END;
$ LANGUAGE plpgsql;

Both _material and _nad are text array columns of a view, as well as all other variables. The whole INSTEAD OF trigger function is too long to reproduce in entirety (almost 200 code rows) - ... in the code sample represent skipped unrelated code.

So why is there the error, and how to solve it?

Without seeing complete code, or a cut-down test case, it's hard to be sure but the error strongly suggests that:

FOREACH _mat
        ^^^^
        this variable is undeclared

StackExchange Database Administrators Q#64046, answer score: 7