Error when importing file (File to import not found or unreadable:)

[amccarthyMatchbook]

Hi,
using Compass 1.0.3 (Polaris) on a mac (OSX 10.10.3)
I have 2 files my application.css.scss and _variables.css.scss.

Gulp compass command works ok, until I try and import _variables.css.scss.
When I run gulp compass when trying to import this file _variables.css.scss I get.

error sass/application.css.scss (Line 1: File to import not found or unreadable: variables.

Have had a good look online and have not found any solid solutions for this error.
Has anyone encountered this before or possibly found a fix for this?

here is the gulp task I am running btw:

gulp.task('compass', function() {
  gulp.src('./source/assets/stylesheets/*.scss')
    .pipe(compass({
      project: path.join(__dirname, './source/assets/stylesheets'),
        sass: 'sass',
        css: __dirname + '/build/source/assets/stylesheets',
        debug: true,
        sourcemap: true
    }))
    .pipe(gulp.dest('app/build/assets'));
});

If anyone could help out it would be greatly appreciated.