Problem changing font size

Hello,

I've been working on a script and have the following line of code, where I create and want to edit the font size of the just created text. However I can't seem to get it work. I can change some of the other text properties, which is why it is stumping me.

var finishText = thisComp.layers.addText(finishTime);     var textProp = finishText.property("Source Text");    var textDocument = textProp.value;    textDocument.resetCharStyle();    textDocument.fontSize = 36;    textProp.setValue(textDocument);

Help appreciated! Thanks!