Ember and bootstrap : failed to build because of SassCompiler

I am working on an ember project and I've decided to integrate Bootstrap by installing ember-bootstrap and using sass preprocessor :

ember install ember-bootstrap
ember generate ember-bootstrap --preprocessor=sass

But the projetc fails to start because of this error :

Build Error (SassCompiler) in 0/ember-bootstrap/_variables.scss:338:31
argument $color of rgba($color, $alpha) must be a color

I am using :

node-sass: 4.9.0
ember-cli: 3.1.4
node: 9.11.1
bootstrap: 4.1.1

For ember-cli-build.js :

var EmberApp = require('ember-cli/lib/broccoli/ember-app');
var nodeSass = require('node-sass'); 

module.exports = function(defaults) {
  var app = new EmberApp(defaults, {
    // Add options here
    sassOptions: {
      includePaths: [
        'bower_components/materialize/sass'
      ],
      nodeSass: nodeSass 
    },

    'ember-bootstrap': {
      'bootstrapVersion': 4,
      'importBootstrapFont': false,
      'importBootstrapCSS': false
    }
  });


  return app.toTree();
};

Any suggestion on where am I going wrong ?