Another backwards MC issue

1

I have a movie clip with a stop command on frame 1 and a stop command on frame 20.

I’m trying to have two separate buttons that tell the MC (onRollOver) to play forward or to play backwards.

The actionscript on button #1-

on(rollOver){
    gotoAndPlay(_currentframe +1);
}

The actionscript on button #2-

on(rollOver){
    this.onEnterFrame = function(){
        gotoAndPlay(prevFrame());
    }
}

My clip works but for some reason when you go back to the first button, it does not play the movie forward anymore.
I have attached a basic example of the .FLA

Thanks in advance!