Chrome not showing sourcemaps for minified CSS generated with gulp

Question

I can't seem to get my sourcemaps to work in Chrome for my minified CSS files. Here's my gulpfile.js:

var gulp = require('gulp'),
    gp_concat = require('gulp-concat'),
    gp_rename = require('gulp-rename'),
    gp_cssmin = require('gulp-cssmin'),
    gp_uglify = require('gulp-uglify'),
    gp_sourcemaps = require('gulp-sourcemaps')
    ;

var src_dir = '../../assets/';
var paths = {
    dir_src: path_dir_src,
    files_css: [src_dir+'css/main.c.src.css'],
    dir_dest: 'src_dir/',
    dir_dest_css: src_dir+'css/'
};

gulp.task('build-css', function(){
    return gulp.src(paths.files_css)
        .pipe(gp_sourcemaps.init())
        .pipe(gp_concat('main.c.raw.css'))
        .pipe(gulp.dest(paths.dir_dest_css))
        .pipe(gp_rename('main.c.css'))
        .pipe(gulp.dest(paths.dir_dest_css))
        .pipe(gp_rename({suffix: '.min'}))
        .pipe(gp_cssmin({sourceMap: true}))
        .pipe(gp_sourcemaps.write('./'))
        .pipe(gulp.dest(paths.dir_dest_css))
        ;
});

When I run this, the sourcemap is created (and it references the original source {...,"sources":["main.c.src.css"],...). However, when the HTML page that references the minified CSS file (main.c.min.css) is opened in Chrome, I don't see the line numbers for the original source file (main.c.src.css).

How can I fix this?