diff --git a/lib/timeline/Graph2d.js b/lib/timeline/Graph2d.js
index 6096f4ca..95cb49d3 100644
--- a/lib/timeline/Graph2d.js
+++ b/lib/timeline/Graph2d.js
@@ -20,7 +20,7 @@ var LineGraph = require('./component/LineGraph');
  */
 function Graph2d (container, items, groups, options) {
   // if the third element is options, the forth is groups (optionally);
-  if (!(groups instanceof Array || groups instanceof vis.DataSet) && groups instanceof Object) {
+  if (!(groups instanceof Array || groups instanceof DataSet) && groups instanceof Object) {
     var forthArgument = options;
     options = groups;
     groups = forthArgument;
diff --git a/lib/timeline/Timeline.js b/lib/timeline/Timeline.js
index fb374076..f5ee3b4a 100644
--- a/lib/timeline/Timeline.js
+++ b/lib/timeline/Timeline.js
@@ -24,7 +24,7 @@ function Timeline (container, items, groups, options) {
   }
 
   // if the third element is options, the forth is groups (optionally);
-  if (!(groups instanceof Array || groups instanceof vis.DataSet) && groups instanceof Object) {
+  if (!(groups instanceof Array || groups instanceof DataSet) && groups instanceof Object) {
     var forthArgument = options;
     options = groups;
     groups = forthArgument;