--- a/cms/app-client/app/models/document.js	Thu Nov 10 15:35:27 2016 +0100
+++ b/cms/app-client/app/models/document.js	Sat Nov 12 17:21:25 2016 +0100
@@ -19,7 +19,7 @@
     subjects: DS.attr({ defaultValue: function() { return []; } }),
 
     duration_ms: DS.attr('number', {
-        defaultValue: function() {
+        defaultValue: () => {
             var self = this;
             var duration = 0;
             Object.keys(this.get('mediaArray')).forEach(function(key) {