TypeError: Error #2007: Parameter text must be non-null.

i have this error and i don't know how to solve it

TypeError: Error #2007: Parameter text must be non-null.

  at flash.text::TextField/set text()

  at glossary_fla::MainTimeline/createbtn()

  at glossary_fla::MainTimeline/letterclick()

this is function createbtn

function createbtn(label:String):MovieClip

{

  var txt:TextField = new TextField();

  txt.defaultTextFormat = new TextFormat('Cooper Black',25,0xFFFFFF);

  txt.text = label;

  txt.autoSize = TextFieldAutoSize.LEFT;

  txt.background = txt.border = false;

  txt.selectable = false;

  var btn:MovieClip = new MovieClip();

  btn.mouseChildren = false;

  btn.buttonMode = true;

  btn.addChild(txt);

  btn.label = label;

  return btn;

}

and this function is letterclick

function letterclick(e:MouseEvent)

{

  myxmllist = new XMLList(myxml.word.(@clas==e.target.name).@wordname);

  for each (var word:XML in myxmllist)

  {

  btn = createbtn(myxmllist);

  btn.x = 600;

  btn.y = 130 + n;

  addChild(btn);

  n +=  50;

  btn.addEventListener(MouseEvent.CLICK,wordclick);

  i++;

  Tweener.addTween(btn, {x:100, scaleX:1, scaleY: 1, time: 2, transition:"easeOut"});

  }

}

could you help me please