Single Page App (SPA) with Durandaljs, Not Able to Load Default Route

I am trying to write a NopCommerce plugin. I have put in my app related files inside plugin's root directory in app named directory. In my shell.js viewmodel I define routes as follows:

define(['durandal/system', 'durandal/plugins/router', 'service/logger','config'],
    function(system, router, logger,config) {

        var shell = {
            activate: activate,
            router: router
        };

        function activate() {
            logger.log('MyProducts Started', null, system.getModuleId(shell), true);
            router.map([
                { route: '', moduleId: 'viewmodels/myProducts', title: 'My Products', nav: true },
                { route: 'searchProducts', moduleId: 'viewmodels/searchProduct', title: 'Search Products', nav: true },
                { route: 'addProducts', moduleId: 'viewmodels/addProduct', title: 'Add Product', nav: true }
            ]).buildNavigationModel();
            return router.activate();
        }

        return shell;
    }
);

Per conventions it should go to first route defined by module: viewmodels/myProducts but I am getting following error:

[viewmodels/shell] MyProducts Started system.js:75
[main] No router found Object
Navigation Complete undefined Object
Uncaught TypeError: Cannot read property 'router' of undefined 

I am banging my head with the well. It is not going to default route (with route:'').

Resolved :)

After much headache, I found out that this is causing the issue:

define([..., 'durandal/plugins/router',...]

When I remove this and it was resolved.

EDIT:- Following is define function inside my main.js file.

define(['durandal/system', 'durandal/app', 'durandal/viewLocator'
                                                            , 'service/logger'],
function (system, app, viewLocator, logger) {
    system.debug(true);

    app.configurePlugins({
        router: true,
        dialog: true,
        widget: true
    });

    app.start().then(function () {
        viewLocator.useConvention();
        app.setRoot('viewmodels/shell', 'entrance');

        //router.mapUnknownRoutes(function (instruction) {
        //    logger.logError('No router found', instruction, 'main', true);
        //});
    });
});