Alpha causes error

/**
 * Copyright DaniilTutubalin ( http://wonderfl.net/user/DaniilTutubalin )
 * MIT License ( http://www.opensource.org/licenses/mit-license.php )
 * Downloaded from: http://wonderfl.net/c/1ETC
 */

package {
    import flash.text.TextField;
    import flash.display.Sprite;
    import flash.display.BitmapData;
    
    public class FlashTest extends Sprite {
        public function FlashTest() {
            var tf:TextField = new TextField();
            tf.width = stage.stageWidth;
            tf.height = stage.stageHeight;
            tf.text = ""; 
            addChild(tf);          
            
            var bmd:BitmapData = new BitmapData(1,1,true,0xFFFFFFFF); 

            for (var i:Number=0; i<256; i++) {
                bmd.setPixel32(0, 0, 0x504845 + (i << 24)); 
                tf.appendText (i+": "+(bmd.getPixel32(0,0) & 0xffffff).toString(16)+"\n")
            }
        }
    }
}