RequireJs 2.0.2 loading underscore 1.3.3 yields null, but OK when loading backbone together
===

I am having the weirdest problem. If I am loading underscore with backbone together using RequireJs 2.0.2, everything works fine. But when only loading underscore, I get null. I have been stuck for hours, can someone help me on this?

Here is the the code that works:

    require.config({
        paths: {
            // JavaScript folders
            libs: "../scripts",

            // Libraries
            jquery: "../scripts/jquery-1.7.1.js ",
            underscore: "../scripts/underscore",
            backbone: "../scripts/backbone",
            mustache: '../scripts/mustache'
        },
        shim: {
            underscore: {
                exports: '_'
            },
            backbone: {
                deps: ["underscore", "jquery"],
                exports: "Backbone"
            }
        }
    });

    require(['jquery', 'backbone', 'underscore', 'scripts/atm/router/router.js'], function ($, Backbone, _, Router) {
        var router = new Router();
    });


And here is the code that tells me _ is null:

    require.config({
        paths: {
            // JavaScript folders
            libs: "../scripts",

            // Libraries
            jquery: "../scripts/jquery-1.7.1.js",
            underscore: "../scripts/underscore"
        },
        shim: {
            underscore: {
                exports: '_'
            }
        }
    });

    require(['jquery', 'underscore'], function ($, _) {
        var i = 100;
    });