Why, when I try to replace an "Int" with an other one in a 2 dimensional Array, I get an "Error #1010: A term is undefined and has no properties."

In my programming class, I need to create a Tiles game with AS3 (like zelda). On a map, the tiles are initialised black in an array, and, after that, they are changing randomly each time the leftpick of a song reach a certain value.

When the tiles change, I need to update the Array with the new value. Actually, I always get an Error #1010: A term is undefined and has no properties.

This is how the tiles are initialised:

    public var tMap: Array = [
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
        [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
    ]; //This is the map. Every tiles are set to 1 (black)
      //this is the array to update
      // Good to know : each tiles are a single frame of a MovieClip

    private var grid: MovieClip; //stock tiles like an array
    private var nbRow: int = 6; //nb of row on the map
    private var nbCol: int = 12; //nb of col on the map
    private var oneTiles: Tiles; // One object Tiles
    private var t: Tiles; //object Tiles


    private function createGrid(): void {
        grid = new MovieClip();
        addChild(grid);

        for (var r: int = 0; r < nbRow; r++) {
            for (var c: int = 0; c < nbCol; c++) {
                t = new Tiles();
                t.x = t.width * c;
                t.y = t.height * r;
                grid.addChild(t);
            }
        }
    }

    //place the tiles on the good frame with tMap
    private function displayTiles(): void {
        var i: int = 0;
        for (var r: int = 0; r < nbRow; r++) {
            for (var c: int = 0; c < nbCol; c++) {
                var t: Tiles;
                t = Tiles(grid.getChildAt(i));
                t.gotoAndStop(tMap[r][c]);
                i++;
            }
        }
    }

This is the function called everytime the leftpeak reach the value

        private function resetTiles(): void {
        //Change tiles randomly

        for (var i: int = 0; i < grid.numChildren; i++) {
            // loop through all child element of a movieclip

            oneTiles = grid.getChildAt(i) as Tiles;
            // Cast display objects to Tiles class.

            oneTiles.getTiles();
            // Call getTiles method
            //30% chance gotoAndStop(1) black tiles
            //70% chance gotoAndStop(2) white tiles

This is the source of the problem : to update the array, I added this to resetTiles(). Whit it, I always get an Error #1010: A term is undefined and has no properties. :

            private var posX: uint = 0; //global
            private var posY: uint = 0; //global

            tMap[posX][posY] = oneTiles.getFrame();

            if(posX == 11 && posY != 5){
                posX = 0;
                posY++;
            }else if(posX == 11 && posY == 5){
                posX = 0;
                posY = 0;
            }else{
                posX++;
            }
            trace(posX);
            trace(posY);

        }

So, where's the problem ? Normaly, with this code, each time a tile is changed, the good tile in tMap shall be updated.

I did some test, and what seems to be the source of the problem is the line tMap[posX][posY] = oneTiles.getFrame(); Still, I can't figure out why

getFrame Method:

public function getFrame():void{
            this.currentFrame;
    }