level._x = undefined - Hvorfor?!

Hej eksperter!
Her er en kode til et spil:

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level = 0;
levels = 0;

// Starting positions (level0x, level0y, ..., ..., levelnx, levelny)
array = new Array(675, 100);

setup();

function setup() {
    createEmptyMovieClip("particles", 1);
    attachMovie("player", "player", 2);
    attachMovie("level"+level, "level", 3);
    attachMovie("interface", "interface", 4);

    // Keys
    left = 65;
    right = 68;
    up = 87;
    down = 83;
   
    // Physics
    gravity = .5;
    friction = .25;

    // Position
    player._x = array[level+levels];
    player._y = array[level+levels+1];
   
    // Current speed
    xspeed = 0;
    yspeed = 0;
   
    // Acceleration
    xacceleration = 1;
    yacceleration = 1;
   
    // Rate of movement
    xrate = .05;
    yrate = .05;
   
    // Maximum speed
    maxxspeed = 50;
    maxyspeed = 50;

    // Level
    level._x = 0;
    level._y = 0;

    setInterval(game, 15);
}

function game() {

    // Move left
    if (Key.isDown(left)) {
        xspeed -= xacceleration;
        if (player._currentframe >= 1 && player._currentframe < 9 ) {
            player.play();
        }

    // Move right
    } else if (Key.isDown(right)) {
        xspeed += xacceleration;
        if (player._currentframe >= 9) {
            player.play();
        }

    // Slow down
    } else if (xspeed < 0) {
        xspeed = Math.min(xspeed+friction, 0);
    } else if (xspeed > 0) {
        xspeed = Math.max(xspeed-friction, 0);
    }

    // Move up
    if (Key.isDown(up)) {
        yspeed += yacceleration;

    // Move down
    } else if (Key.isDown(down)) {
        yspeed -= yacceleration+gravity;

    // Fall
    } else {
        yspeed -= gravity;
    }

    // Keeping speed above minumum and below maximum
    xspeed = Math.max(Math.min(xspeed, maxxspeed), maxxspeed*-1);
    yspeed = Math.max(Math.min(yspeed, maxyspeed), maxyspeed*-1);

    // Collission
    if (level.hitTest(player._x, player._y, true)) {
        attachMovie("explosion", "explosion", 10);
        explosion._x = player._x;
        explosion._y = player._y;
        fscommand("quit");
    } else {

    // Positioning
        player._x += xrate*xspeed;
        player._y -= yrate*yspeed;
    }
}

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Men den registrerer aldrig hitTest som true; hvorfor ikke?
Jeg har testet med trace, og level._x = undefined.
Hvordan kan dette lade sig gøre???