1061: Call to a possibly undefined method startDrag through a reference with static type flash.display:SimpleButton.

I've got a problem with this and I couldn't understand it.

Scene 1, Layer 'textentry', Frame 2, Line 7 1061: Call to a possibly undefined method startDrag through a reference with static type flash.display:SimpleButton.

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I'm trying to do a Drag&Drop by looking a code in the internet.

addChild(mc1);

mc1.x = 0;

mc1.y = 0;

mc1.addEventListener(MouseEvent.MOUSE_DOWN, start_move);

function start_move(evt:MouseEvent):void {

mc1.startDrag();

}

mc1.addEventListener(MouseEvent.MOUSE_UP, stop_move);

function stop_move(e:MouseEvent):void {

mc1.stopDrag();

}

Thanks for your advice.