Loading variable data from an external text doc

1

Can anyone spot the error here that is giving me a TypeError: Error #2007: Parameter text must be non-null.

The text field captions are in a file named captions.txt in the same root folder.

Still learning!
thanks

var myURLLoader:URLLoader = new URLLoader();
var myURLRequest:URLRequest = new URLRequest("captions_tuesday.txt");
myURLLoader.dataFormat = URLLoaderDataFormat.VARIABLES
myURLLoader.load(myURLRequest);

myURLLoader.addEventListener(Event.COMPLETE, dataOK);


function dataOK(myevent:Event):void
    {
    textfield1.text=myURLLoader.data.caption1;
    textfield2.text=myURLLoader.data.caption2;
    textfield3.text=myURLLoader.data.caption3;
    }