seajs模块合并

Question

'concat' : {

js : {
    options: {  
          include: 'all'  
    },
     files : {
            'dist/index.js' : ['dest/index.js','lib/jquery.js','lib/jquery.cookie.js']
    }
}

}
报错 ：
$ grunt concat
Running "concat:js" (concat) task
Warning: grunt.util._.contains is not a function Use --force to continue.
请问什么原因导致的