How to add an expression to the start, and end properties of a text animator range selector?

So I have a code trying to add an expression to these properties, but the error comes back saying they are hidden?

Any help is appreciated, here's my code currently:

var animator = app.project.activeItem.selectedLayers[0].Text.Animators.addProperty("ADBE Text Animator");
var selector = animator.property("ADBE Text Selectors").addProperty("ADBE Text Selector");
selector.advanced.units.setValue(2);
var startExpression=("0");
var endExpression=("10");
selector.start.expression=startExpression
selector.end.expression=endExpression;