Set dynamic text in mc from parent

This is probably easy but I’m missing something… I’m trying to assign a text string to a dynamic text field within a movieclip.

In my FLA’s library I have my movie clip “mc” (linkage = MC) and mc has this code in it:

// internal mc code
var tf:TextField = new TextField();
addChild(tf);

In Frame 1 I have:

var mc = new MC();
addChild(mc);
mc.tf.text = "here is some text.";

This produces the following error:
[FONT=“Courier New”]TypeError: Error #1009: Cannot access a property or method of a null object reference.[/FONT]

What am I missing?