Grunt compass:dist task failing due to Invalid CSS in bootstrap-sass-official/assets/stylesheets/bootstrap/_variables.scss

[zishon89us]

grunt build is failing by yielding following error:

error app/styles/directive.scss (Line 369 of
bower_components/bootstrap-sass-official/assets/stylesheets/bootstrap/_variables.scss:
Invalid CSS after "... floor(math": expected ")", was
".div($grid-gutt...")

Gruntfile.js has compass and build task like below:

grunt.initConfig({

		....
		
		compass: {
			options: {
				sassDir: '<%= yeoman.app %>/styles',
				cssDir: '.tmp/styles',
				generatedImagesDir: '.tmp/images/generated',
				imagesDir: '<%= yeoman.app %>/images',
				javascriptsDir: '<%= yeoman.app %>/scripts',
				fontsDir: '<%= yeoman.app %>/styles/fonts',
				importPath: './bower_components',
				httpImagesPath: '/images',
				httpGeneratedImagesPath: '/images/generated',
				httpFontsPath: '/styles/fonts',
				relativeAssets: false,
				assetCacheBuster: false,
				raw: 'Sass::Script::Number.precision = 10\n'
			},
			dist: {
				options: {
					generatedImagesDir: '<%= yeoman.dist %>/images/generated'
				}
			},
			server: {
				options: {
					sourcemap: true
				}
			}
		},
		
		concurrent: {
			server: [
				'compass:server'
			],
			test: [
				'compass'
			],
			dist: [
				'compass:dist',
				'imagemin',
				'svgmin'
			]
		},
 		....
	});

grunt.registerTask('build', [
		'concurrent:dist',
	]);

package.json includes:

"grunt-contrib-compass": "^1.0.0",

bower.json includes:

"bootstrap-sass-official": "^3.2.0",

Other information:

OS: Linux Ubuntu 18.10

Node: v8.17.0

NPM: v6.13.4

Ruby: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux-gnu]

Compass: Compass 1.0.3 (Polaris)