Previous function doesn't go all the way back to the first frame

[Jip-Hop]

I think the -1 in the code below is causing the problem. When using the previous button or the comma key on the keyboard in viewer.html the prev function is called. However, it only goes back as far as frame 2. I'm not sure but removing the -1 seems to correct the issue for me.

this.prev = function() {
    self.pause();

    if(_point_index-1 !== 0) {
        _point_index--;
        drawMaterial();
    }
};