is array bug? (array[][])[] = NULL

Hello

   I am not sure so decrease of array dimensions is regular operations.
But following code dont make syntax error.

create or replace function xxx() returns bool as $$
declare v1 varchar[]; v2 varchar[][];
begin
  v2 := array[array[1,2], array[3,4]];
  raise notice 'v2 %', v2;
  v1 := v2[1];
  raise notice 'v1 %', v1;
  return 't';
end; $$ language plpgsql;

I expect v1 = [1,2], but v1 is null.
What is wrong?

Regards
Pavel Stehule