Shell not loading

[bittrex-bill]

Hi,

I use the following directory structure with my application.

App
/ Shell / Viewmodels / Shell.js
/ Shell / View / Shell.html
/ main.js

My main.js simply calls
app.setRoot('shell/viewmodels/shell', 'entrance');

My gulpfile is very basic:
gulp.task('durandal', function () {
durandal({
baseDir: 'app', //same as default, so not really required.
main: 'main.js', //same as default, so not really required.
output: 'main-built.js', // custom build file
almond: true,
minify: false,
verbose: true,
durandalDynamicModules: true
})
.pipe(gulp.dest('scripts'));
});

After running gulp durandal, my main-built.js file throws an error when trying to resolve shell/viewmodels/shell

I see it has been included in the main-built.js file:

define('Shell/ViewModels/shell',['plugins/router', 'durandal/app', ], function (router, app) {
...
}

Any thoughts on why I'm hitting the exception "undefined missing shell/viewmodels/shell" in main?
it is occurring at this line:
throw new Error(name + ' missing ' + depName);

Any help is appreciated.
Bill