Assigning elements in a list of lists

I can't understand why I am not able to assign an element in a list of 
lists like in

--------
(35) -> A:=[[0,0,0,0]]
   (35)  [[0,0,0,0]]   	Type: List List NonNegativeInteger

(36) -> A(1,1)
    (36)  0           	Type: NonNegativeInteger

(37) -> A(1,1):=1

    The form on the left hand side of an assignment must be a single 
variable, a Tuple of variables or a reference to an entry in an object
supporting the setelt operation.
--------

whereas if I declare A as a matrix it works:

--------
(43) -> A:=[[0,0,0,0]] :: Matrix INT
    (43)  [0  0  0   	Type: Matrix Integer

(44) -> A(1,1):=1
    (44)  1		Type: PositiveInteger

(45) -> A
    (45)  [1  0  0  0]	Type: Matrix Integer
--------

Can anybody please explain me what is wrong in the first esample?

Thank you very much

Fabio