How to change a layout label text in code

I have a label in my layout.xml file, defined as follows:

	<label id = "header">
		<param name="text">Not yet set</param>
		<param name="x">0</param>
		<param name="y">vv_icons.height</param>
		<param name="width">dc.getWidth()</param>
		<param name="height">45</param>
	</label>

I want to set the label text in my code.  I thought the following should work, but I get the error "Could not find symbol 'text'.  Why is this?

    function onLayout(dc) 
    {
        setLayout(Rez.Layouts.AccelTarePage(dc));
        var drw;        
        drw = self.findDrawableById("header");
        drw.text = "My Header";
    }