angularjs - Underscore mixin error -

i trying learn grunt tasks below example https://github.com/song-rate-mvc/angular-song-rate/tree/0.0.3

after command npm install, bower install during grunt dev below error "referenceerror: can't find variable: _" in app.js file

the error occurs in file app.js @

angular.module("myapp", [   'myapp.filters',   'myapp.services',   'myapp.directives',   'myapp.controllers' ]); //error occurs in below line. _.mixin(_.string.exports()); 

am missing anything...