diff --git a/instance/main.js b/instance/main.js
index ead4ab6bbf608db0c56bbb2b73206526eada9831..f2f74989ac8ee53f685b6b84dae792ff2b830942 100644
--- a/instance/main.js
+++ b/instance/main.js
@@ -301,8 +301,8 @@ function onDragStartQueue(event) {
 
 function onDragEnter(event) {
     if (event.currentTarget) {
-        if (leavedElement != event.currentTarget) {
-            leavedElement.style.borderTop = '';
+        if (leavedElement != event.currentTarget && leavedElement) {
+            leavedElement.style.borderTop = ''; // style is undefined !
         }
         event.currentTarget.style.borderTop = '3px solid grey';
         dragCounter++;