Arrays

Hello,

I'm trying to use a bidimensional array and I'm having the following
problem:

students=# UPDATE years SET y_grade[1] = '{2,3,4,5,6,7}';
ERROR:  pg_atoi: error in "{2,3,4,5,6,7}": can't parse "{2,3,4,5,6,7}"

I've declared y_grade as a bidimensional array (y_grade smallint[][]), but
still, I get this when I describe the table:

students=# \d years
                          Table "years"
   Column   |    Type    |               Modifiers
------------+------------+----------------------------------------
 y_id       | integer    | not null default nextval('Y_ID'::text)
 y_year     | smallint   |
 y_student  | smallint   |
 y_grade    | smallint[] |
 y_absences | smallint   |
Primary key: years_pkey

(notice it's only a onedimensinal array)
I don't know if that's how it's supposed to be, but even if it is, my query
isn't working.
I'm not trying to enlarge the array, I read in the docs you can't do that:
students=# select * from years;
 y_id | y_year | y_student |            y_grade            | y_absences
------+--------+-----------+-------------------------------+------------
    4 |      1 |         1 | {{0,0,0,0,0,0},{0,0,0,0,0,0}} |
(1 row)

So, any idea why I'm getting this error?