Sprite trying to remove itself from the stage is throwing errors

1

I’m trying to have a bullet Sprite remove itself from the stage once it is off the stage, but Flash keeps throwing #1009 errors at me. I’ve looked over the code several times and don’t understand what’s wrong.

Bullet code:

package {
	import flash.display.Sprite;
	import flash.display.MovieClip;
	import flash.events.*;
	
	public class Bullet extends Sprite {
		private var bullet_direction;
		private var bullet_speed;
		public function Bullet(xLoc:Number, yLoc:Number, angle:Number, speed:Number) {
			x = xLoc;
			y = yLoc;
			bullet_direction = angle;
			bullet_speed = speed;
			addEventListener(Event.ENTER_FRAME, BulletMovement,false,0,true);
		}

		public function BulletMovement(e:Event) {
			y -= bullet_speed * Math.sin(bullet_direction);
			x += bullet_speed * Math.cos(bullet_direction);
			if ( x > 500 || x < -20 || y > 660 || y < -20 ) {
				DestroyBullet();
			}				
		}

		public function DestroyBullet():void {
			removeEventListener(Event.ENTER_FRAME, BulletMovement);
			(stage as MovieClip).removeChild(this);
		}
	}
}

Error message:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Bullet/DestroyBullet()
	at Bullet/BulletMovement()

Any help would be much appreciated.