rem-calc of Founation in WebStorm

Hi.

I'm working with SASS Foundation via gulp and gulp-sass.

This is my gulp task: 

gulp.task('scss', function () {
    return gulp.src('assets/scss/*.scss')
        .pipe(sass({includePaths: ['node_modules/foundation-sites/scss/'], outputStyle: 'compressed'}).on('error', sass.logError)) //foundation includes
        .pipe(gulp.dest('assets/css'))
        .pipe(connect.reload());
});

When I use rem-calc() in .scss files I've got error message:

Also there is no autocomplete for rem-calc() func.

How can I fix this small problem?