why does my script fail when it hits the appendChild?

Question

When I call my function it fails when it hits the appendChild I have no idea why:

I create my function:

Auto.prototype.create = function(container){
   this.contain = container;

   var mask = document.createElement("div");
   mask.style.position = "absolute";

   this.contain.appendChild(mask);

When I call the create function, it fails when it hits the appenChild do I need to call that too? because I thought that calling the function was all I needed.