I keep getting an error when I try to set the text of a shape to xml format

NRectangleShape myRectangle = new NRectangle(100, 100, 200, 100);
myRectangle.Text = "htmltext";
NTextStyle textShapeStyle = new NTextStyle();
myRectangle.Style.TextStyle.TextFormat = TextFormat.XML;//keep getting an error here
myRectangle.Style.TextStyle = textShapeStyle;

I don't want to apply xml format to the whole document, only to that shape