From ae5acb65471d8c6cd77929e1ee684ec14d6a518c Mon Sep 17 00:00:00 2001
From: Jonathan Nielsen <jmnetus@familysearch.org>
Date: Fri, 4 Oct 2013 13:06:25 -0600
Subject: [PATCH] Add similar refPoint validity check to zoomBy

---
 src/viewport.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/viewport.js b/src/viewport.js
index 5c4c9f2d..3fa7077d 100644
--- a/src/viewport.js
+++ b/src/viewport.js
@@ -536,7 +536,7 @@ $.Viewport.prototype = {
      * @return {OpenSeadragon.Viewport} Chainable.
      */
     zoomBy: function( factor, refPoint, immediately ) {
-        if( refPoint ) {
+        if (refPoint instanceof $.Point && !isNaN(refPoint.x) && !isNaN(refPoint.y)) {
             refPoint = refPoint.rotate(
                 -this.degrees,
                 new $.Point( this.centerSpringX.target.value, this.centerSpringY.target.value )