diff --git a/static/js/viewer.js b/static/js/viewer.js
index d21d068..4708e32 100644
--- a/static/js/viewer.js
+++ b/static/js/viewer.js
@@ -118,7 +118,7 @@ $(function() { sio = (function() {
 
     $(window).resize(update);
 
-    sio.onMouseDown = function() {
+    sio.onMouseDown = function(event) {
 
         var x = event.offsetX / canvasPdf.width;
         var y = event.offsetY / canvasPdf.height;