diff --git a/examples/network/15_dot_language_playground.html b/examples/network/15_dot_language_playground.html
index c4247543..c79ee3c3 100644
--- a/examples/network/15_dot_language_playground.html
+++ b/examples/network/15_dot_language_playground.html
@@ -80,7 +80,8 @@
 </table>
 
 <script type="text/javascript">
-  var network, data;
+  var network = null;
+  var data = null;
 
   var btnDraw = document.getElementById('draw');
   var txtData = document.getElementById('data');