I have had the same problem, but with a bit of a twist.
I'm using MovieClip(root) in a private function of a class file bit even though I can trace that MovieClip(root) is the main time line it gives me an error about referencing a null object. the class file is directly attached to the movie clip on the stage, I have 3 frame an intro with a button to send the user to the second frame where the movie clip is and all the game elements are dynamically added to the MC then in the class file the private function among other things checks if teh game has ended and if so sends the user to the third frame which has the game over text and a button that they can click to play another game (get sent back to the second frame) but as soon as the variable that checks for the end of the game reaches zero the if condition is meet in there it just has a line MovieClip(root),gotoAndStop("gameover"); which is the third frame, if I comment out this line there is no error message but of cause no functionality either to go to the third frame. Does any one know what may be happening here as this should work by all accounts. Big mystery for me and I would like to understand what I have done wrong here.