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 )