Can't hide a scrollbar?

1 

function makeScroll(textToScroll) {
	textToScroll.createClassObject(mx.controls.UIScrollBar, "my_sb", textToScroll.getNextHighestDepth());
	// Set the target text field for the scroll bar.
	my_sb.setScrollTarget(textToScroll);
	// Size it to match the text field.
	my_sb.setSize(16, textToScroll._height); 
	// Move it next to the text field.
	my_sb.move(textToScroll._x + textToScroll._width, textToScroll._y);
	if(textToScroll.textHeight > textToScroll._height) {
		trace("scroll");
		textToScroll.my_sb._visible = true;
	} else {
		trace("no scroll");
		textToScroll.my_sb._visible = false;
	}
}
//usage
test.contentTxt.text = "challenge, there is evolution. In response to the the incredible challenges faced ....";

makeScroll(test.contentTxt);

but i still see the background of the scrollbar my_sb

any ideas