gulp-sourcemaps: Node V6 error - 'Path must be a string. Received undefined'

Receiving error ‘Path must be a string. Received undefined’ when running with Node V6. This Gulp task worked fine with Node V5.11.0. Using gulp-sourcemaps v2.0.0-alpha

gulp.task('stylus', function () {
     return gulp.src('src/**/*.styl')
        .pipe(flatten())
        .pipe(sourcemaps.init())
        .pipe(stylus({use: rupture()}))
        .pipe(plumber())
        .pipe(uncss({html: ['src/**/*.html']}))
        .pipe(cleanCSS())
        .pipe(postcss(processors))
        .pipe(concat('css.min.css'))
        .pipe(sourcemaps.write('/maps'))
        .pipe(gulp.dest('./dist/css'))
        .pipe(reload({stream: true, match: '**/*.css'}));
});

Task works fine when I comment out both sourcemap lines.