Accessing root from a dynamic clip

programming flash
1

I have looked over several posts and can’t seem to find this answer.
Basically I have a _mc that I am calling in from the library. When I get to the end of the clip I want it to advance the root time line to frame 3.
This is the code I have at the end of the _mc
MovieClip(root).gotoAndStop(3);

This is the error I get :
Error #1009: Cannot access a property or method of a null object reference.
at introScene/frame91()
TypeError: Error #1034: Type Coercion failed: cannot convert flash.display::Stage@44370ba1 to flash.display.MovieClip.
at introScene/frame91()

Thanks for the help…