Error 1009: Very Very Basic

Hey there. I recently started to learn ActionScript 3.0 about 3 days ago. I am trying to do a very simple task in which my animation moves across the X-axis by 10 in every frame I put my code in. Only problem is, somehow my coding is wrong for my first frame and I get an error message 1009 for a null object. I am learning off of tutorials and it doesn't make sense to me how I could be getting this problem when I am copying pretty much the exact same code as the person in the tutorial. If anyone has any advice on why I may be experiencing this problem please help! I was just getting really into all of this and I would like to continue doing it for a long time.

If anyone could help that would be much appreciated!

Thank you!

P.s. here's the code I used in the first frame...

rockFly();

function rockFly():void {

  rockguy.x += 10;

}