diff --git a/misc/tabledrag.js b/misc/tabledrag.js
index 16e9bd6..836568f 100644
--- a/misc/tabledrag.js
+++ b/misc/tabledrag.js
@@ -581,8 +581,9 @@ Drupal.tableDrag.prototype.dropRow = function (event, self) {
  */
 Drupal.tableDrag.prototype.mouseCoords = function (event) {
 
-  // Match both null and undefined, but not zero, by using != instead of !==.
-  if (event.pageX != undefined && event.pageY != undefined) {
+  // Match both null and undefined, but not zero, by using != null.
+  // See https://stackoverflow.com/questions/2647867/how-to-determine-if-variable-is-undefined-or-null
+  if (event.pageX != null && event.pageY != null) {
     return {x: event.pageX, y: event.pageY};
   }
 
@@ -590,21 +591,21 @@ Drupal.tableDrag.prototype.mouseCoords = function (event) {
   // version 1.11.1; between versions 1.7 and 1.11.0 pointer events have the
   // pageX and pageY properties undefined. In those cases, the properties must
   // be retrieved from the event.originalEvent object instead.
-  if (event.originalEvent && event.originalEvent.pageX !== undefined && event.originalEvent.pageY !== undefined) {
+  if (event.originalEvent && event.originalEvent.pageX != null && event.originalEvent.pageY != null) {
     return {x: event.originalEvent.pageX, y: event.originalEvent.pageY};
   }
 
   // Some old browsers do not support MouseEvent.pageX and *.pageY at all.
   // See https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/pageY
   // For those, we look at event.clientX and event.clientY instead.
-  if (event.clientX == undefined || event.clientY == undefined) {
+  if (event.clientX == null || event.clientY == null) {
     // In some jQuery versions, some events created by jQuery do not have
     // clientX and clientY. But the original event might have.
     if (!event.originalEvent) {
       throw new Error("The event has no coordinates, and no event.originalEvent.");
     }
     event = event.originalEvent;
-    if (event.clientX == undefined || event.clientY == undefined) {
+    if (event.clientX == null || event.clientY == null) {
       throw new Error("The original event has no coordinates.");
     }
   }