Connecting Array and Frames

programming flash
1

I am really starting to hate actionscript 3.0…the problem is in my movie i have a question, where the user needs to write an anwer in the input text field, and if the answer is correct he should go to a new frame, but if it’s wrong he is sent again to a different frame!..the code I’m using is below. As I publish the movie, flash shows that everything is fine, everytime you write a wrong answer it takes the user to the frame it supposed to take, but it if you write the 2nd and 3rd correct answer (“two”, “three”) it also takes to the “back frame”, where it should take it to the “next” frame, it works when you write the 1st correct answer thought!..
and it keeps sending me this message:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at project_fla::MainTimeline/answer() Attach Code


stop();
var users:Array = ["one", "two", "three"];


enter_btn.addEventListener(MouseEvent.CLICK, answer);

function answer(event:MouseEvent):void
{
	for(var i:Number = 0; i < users.length; i++)
	{
		if(users* == input_txt.text)
		{
			gotoAndPlay("next");
		}
		else
		{
			gotoAndPlay("back");
	}
}