r.js doesn't optimize requirejs implicitly

This is my r.js build config file.

If i omit the requirejs module in the modules option, it doesn't optimize requirejs by default. If i add requirejs explicitly (as shown below) it optimizes fine. Is this expected behaviour what am i doing wrong?

var config = {
    appDir: paths.dev_dist,
    baseUrl: 'scripts/lib',
    mainConfigFile: paths.src.common + '/scripts/common.js',
    skipDirOptimize: true,
    modules: [
        {
            name: 'requirejs',
            include: ['requirejs']
        },
        {
            name: 'app/common',
            include: ['app/main']
        }
    ]
};