Flash Error #1009 for a Button - actionscript-3

Question

I'm new to AS3, and I keep getting Error #1009 when I try to add a button. From some reading I did I gather it is when I reference something I think exists but that really doesn't, except I have added a correct instance_name, and this did not happen for any of my other buttons.
Below is my code:
stop();
//buttons
Erhu_btn.addEventListener(MouseEvent.MOUSE_DOWN, Erhu_func);
Flute_btn.addEventListener(MouseEvent.MOUSE_DOWN, Flute_func);
Guzheng_btn.addEventListener(MouseEvent.MOUSE_DOWN, Guzheng_func);
Ruan_btn.addEventListener(MouseEvent.MOUSE_DOWN, Ruan_func);
Yangqin_btn.addEventListener(MouseEvent.MOUSE_DOWN, Yangqin_func);
Compose_btn.addEventListener(MouseEvent.MOUSE_DOWN, Compose_func);
Intro_btn.addEventListener(MouseEvent.MOUSE_DOWN, Intro_func);
//function definitions
function Erhu_func(event:MouseEvent):void {
gotoAndStop("Erhu");
}
function Flute_func(event:MouseEvent):void {
gotoAndStop("Flute");
}
function Guzheng_func(event:MouseEvent):void {
gotoAndStop("Guzheng");
}
function Ruan_func(event:MouseEvent):void {
gotoAndStop("Ruan");
}
function Yangqin_func(event:MouseEvent):void {
gotoAndStop("Yangqin");
}
function Compose_func(event:MouseEvent):void {
gotoAndStop("Compose");
}
function Intro_func(event:MouseEvent):void {
gotoAndStop("Intro");
}
All the buttons work except for the Intro_btn. I know it's probably a really simple mistake, but I can't figure it out, and I would really appreciate any light you could shed on my problem!