"Expected identifier, string or number" in IE7

I've just updated to the latest dev version so my maps will work in IE, and while they are now working in IE8, IE7 displays this error and no map:

Message: Expected identifier, string or number

Line: 18

Char: 5

Code: 0

URI: http://www.ava.asn.au/sites/www.ava.asn.au/modules/ava_module/layer-hand...

The file in question is part of a custom module I wrote to add my own map layer. However, it works fine in IE8, so I'm hoping someone can help me fix it for IE7 too:

OL.Layers.TMS = function(layerOptions, mapid) {

  // Avoid pink tiles

  OpenLayers.IMAGE_RELOAD_ATTEMPTS = 3;

  OpenLayers.Util.onImageLoadErrorColor = "transparent";

  return new OpenLayers.Layer.TMS(
    "TMS Layer",
    "http://www.ava.asn.au/sites/www.ava.asn.au/files/floor_plan/",
    {
      maxExtent: new OpenLayers.Bounds(0.0, 0.0, 1508.0, 1524.0),
      numZoomLevels: 4,
      type: 'jpg',
      getURL: overlay_getTileURL,
    } // <-- This is the line referred to by the error...
  );
}

function overlay_getTileURL(bounds) {

  var res = this.map.getResolution();

  var x = Math.round((bounds.left - this.maxExtent.left) / (res * this.tileSize.w));

  var y = Math.round((bounds.bottom - this.maxExtent.bottom) / (res * this.tileSize.h));

  var z = this.map.getZoom();
  if (x >= 0 && y >= 0) {

    return this.url + z + "/" + x + "/" + y + "." + this.type;

  } else {

    return "http://www.maptiler.org/img/none.png";

  }

}