Calling PointBasedClustering,js file from custom,js file is causing an issue

Question

I am calling PointBasedClusteringmodule from a function in custom.js file

GetMap function is called on page load and the code resides in custom.js file.

function GetMap() {

        // Initialize the map
    map = new Microsoft.Maps.Map(document.getElementById("MapHolder"),
               {
                   credentials: "xxxxxxxxxxxxxxx", zoom: 2,
                   mapTypeId: Microsoft.Maps.MapTypeId.road, showDashboard: true,
                   center: new Microsoft.Maps.Location(41.021429, 21.337973)
               });
    //Nate: 3/26/15: for clustering pushpins
    Microsoft.Maps.registerModule("PointBasedClusteringModule", "~/PointBasedClustering.js");

    Microsoft.Maps.loadModule("PointBasedClusteringModule", { callback: moduleLoaded });

    Microsoft.Maps.Pushpin.prototype.title = null;
    Microsoft.Maps.Pushpin.prototype.description = null;

    //Getlocations function moved to moduleloaded function

}

The PointBasedClustering.js file as follows. The error is coming after the module is loaded at

Microsoft.Maps.moduleLoaded('PointBasedClusteringModule');

Unhandled exception at line 447, column 1 in http://localhost:52723/Scripts/PointBasedClustering.js

0x800a1391 - JavaScript runtime error: 'Microsoft' is undefined

Any ideas on what is causing this.

Thanks

Nate