Can you explain why this button isn't working? Flash CS3?

I'm making a site in Flash CS3. It's a pretty basic one but I've run into a problem which I can't get my head around.

I have a load of buttons within Movie Clips that work fine, but sometime after doing the initial programming I decided to add a new button onto each page linking back to the homepage and it just won't seem to work properly.

I really don't understand why because they coding I've done for it is the same as the ones that do work. I already have two separate buttons linking to the homepage, so all I figured I'd have to do is put this one next to them in the action script - but this JUST DOESN'T WORK. and I really can't figure out why.

Here's the code

header_mc.home_btn.addEventListener(MouseEvent.CLICK, gohome); - WORKS

header_mc.logo_btn.addEventListener(MouseEvent.CLICK, gohome); - WORKS

about_mc.back_btn.addEventListener(MouseEvent.CLICK, gohome); - DOESN'T WORK

function gohome (event:MouseEvent):void{

gotoAndStop(1);

}

I have chekced all the syntax and the instance names and the targeting and that's all fine. The error is: TypeError: Error #1009: Cannot access a property or method of a null object reference.

at Charlotte4_fla::MainTimeline/Charlotte4_fla::frame1()

The button I'm having a problem with is not on Frame 1, it is on frames 2-5. To check if this was the problem I put the button in the same movie clip as the working ones on each frame - but it still wouldn't work.

That was literally the only difference (I can find) between the new button and the old working ones - so I really don't get it - I can't see what is going wrong at all. Any Ideas?

at the very least can someone just explain what the error message means? What does "null object reference" mean? I've found loads of people online explaining how to fix particular occurences of this error but it seems to come up so often that a tonne of things must trigger it. Maybe if I understand the error I can figure it out myself!

Sorry this was pretty long, I hope you read it all without wanting to punch me in the throat.

Cheers