How to edit a text Master Property via ExtendScript?

Hi,

This seems basic, but I can't figure it out. I can't change the value of my text Master Property, via Extendscript. I tried treating the property the same way I would treat the Source Text property of a text layer, but After Effects doesn't like this.

var newText = new TextDocument("This is new text.");

var myLayer = myComp.layer("Title Text"); //This is my comp with the master properties.

var myLayer2 = myComp.layer("Text 1"); //This is my text layer.

var myProp = myLayer.masterProperty("Text");

var myProp2 = myLayer2.sourceText;

myProp2.setValue(newText); //This is changing the text of the text layer and works.

myProp.setValue(newText); //This is changing the text of the master property and doesn't work.

The error I get in Extendscript is "After Effects error: invalid text layer."

I would appreciate any help.