Scene 1, Layer 'as', Frame 1, Line 24 1050: Cannot assign to a non-reference value.

I'm trying to make a flash site with that automatically resizes to fit the users browser. the code i used is :

function init()
{

    stage.align=StageAlign.TOP_LEFT;
    stage.scaleMode=StageScaleMode.NO_SCALE;

    stage.addEventListener(Event.RESIZE, updateSize);
    stage.dispatchEvent(new Event(Event.RESIZE));

    myBackground.x = 0;
    myBackground.y = 0;

}

init();

function updateSize(e:Event)
{

    myBackground.width = stage.stageWidth;
    myBackground.height = stage.stageHeight;

    myContent.x = stage.stageWidth/2 = myContent.width/2;
    myContent.y = stage.stageHeight/2 = myContent.height/2;
}

******i keep getting the error "cannot assign to a non-reference value.*******