Grunt Copy Flatten not working as expected

I have a directory structure as follows:

source/
    libraries/
        d3.js
        lodash.js
        //etc

I have grunt-copy setup as follows:

copy: {
  main: {
    files: [
      {
        src: ["source/libraries/*.js"], 
        dest: "build/", 
        flatten: true
      }

I expect it to flatten the output into build, so that I will have

build/
    d3.js
    //etc

Instead, I get a reproduction of the original directory structure in build:

build/
    source/
        libraries/
            d3.js
            //etc

What gives? Am I not using flatten properly?