requirejs使用Headroom.js出现Headroom is not defined，有没有大神知道什么问题

shim:{
'headroom': {
exports:'headroom'
}
}
//定义模块
define(['headroom'],function () {
console.log('Module headroom loaded.');

var headroom = new Headroom('nav', {
"tolerance": 5,
"offset": 205,
"classes": {
"initial": "animated",
"pinned": "slideDown",
"unpinned": "slideUp"
}
});
});