gotoAndStop + textfield 2x?

Following situation;
A library MC with class name zzz, containing 2 keyframes,
1st keyframe a dynamic text field instance name lettervalue Arial,
2nd keyframe a dynamic text field instance name lettervalue Times

now placing actionscript in main timeline 1st frame

var let = new zzz();
addChild(let);
let.gotoAndStop(2);
let.lettervalue.text = "some text";

Gives error message. If I remove either
let.gotoAndStop(2);
or
let.lettervalue.text = “some text”;
Then it doesn`t give any error.

Also if let.lettervalue.text = “some text”; is placed in a following frame it does work. But I need to perform one instantly after the other.

Anybody any idea why or how to solve it?

(ps: it`s a simplified situation, actual one has more then 2 frames with dynamic textfield and gotoAndStop depends on input)
P.