How come I can't add a TextField to a MovieClip?

programming flash
1

I’ve got a MovieClip on the MainTimeline: mc_back

I want to dynamcially add textfields to it:

txt:TextField = new TextField();
txt.Height = 20;
txt.Width = 60;
txt.text = ‘This is Some Text’

mc_back.addChild(txt);

It’s not working. No errors, no nothing, but the textFields don’t show up when I run it.