How to access rectangular array values of data model fields in Django

Consider this model:

from django.contrib.postgres.fields import ArrayField
class Board(models.Model):
    pieces = ArrayField(ArrayField(models.IntegerField()))

As a example, suppose that this is one of the values:

[[1,2], [3,4]]

I tried to access the values of this nd array, but I got list index out of range error.

            Board= Board.objects.all()
            final_list = [
            { "pieces_one": row.pieces[0][0],
              "pieces_four": row.pieces[1][1]
            } for row in Board
        ]

Any suggestion on accessing nd array filed's inner values? Of course, I am using Postgres as my database.

UPDATE: Actually the problem was another thing, and for accessing the inner values of a rectangular or any nd array, this method works fine with Django and Postgres.