Grunt CSS min - Issue with minifying different CSS files

Question

I'm having issue with minifying multiple CSS files using grunt cssmin I'm not looking to minify all files into single file. I would like to have the files having same name with min.css extension.

My gruntfile.js is as follows.

module.exports = function (grunt) {
    grunt.initConfig({

    cssmin: {
        target: {
            files: [{ 
                src: ['assets/css/*.css', '!assets/css/*.min.css'],  // source files mask
                dest: 'assets/css/',    // destination folder
                expand: true,    // allow dynamic building
                flatten: true,   // remove all unnecessary nesting
                ext: '.min.css'   // replace .css to .min.css
            }],
            /* BELOW IS ONLY A TRICK USED TO MINIFY THE FILES SKIPPED BY THE ABOVE */
            /*files: [{ 
                src: ['assets/css/home.css', 'assets/css/institutions.css', 'assets/css/form-elements.css'],  // source files mask
                dest: 'assets/css/',    // destination folder
                expand: true,    // allow dynamic building
                flatten: true,   // remove all unnecessary nesting
                ext: '.min.css'   // replace .css to .min.css
            }],*/
        }
    }
});

grunt.loadNpmTasks('grunt-contrib-cssmin');
grunt.registerTask('default', [ 'cssmin' ]);
};

As per the code (except the commented out part), it is suppose to minify all the css files. But for some reason it skips few css files from the directory. To minify those, I have to comment out the first files[{}] and uncomment the later.

It doesn't work when both files[{}] are uncommented. I'm clueless on why its happening.

My Nodejs version is: v0.10.25

Thanks in advance.

Note: I used to get warnings & task aborts on grunt cssmin due to node version issue and got it fixed by downgrading nvm version to nvm v0.10.39. Thanks to blindMoe for pointing the solution