diff --git a/src/tiledimage.js b/src/tiledimage.js
index 8145a4d3..95ac4f3c 100644
--- a/src/tiledimage.js
+++ b/src/tiledimage.js
@@ -423,8 +423,8 @@ $.extend($.TiledImage.prototype, $.EventSource.prototype, /** @lends OpenSeadrag
      * @returns {OpenSeadragon.Point} The TiledImage's content size, in window coordinates.
      */
      getSizeInWindowCoordinates: function() {
-        var topLeft = this.viewport.imageToWindowCoordinates(new $.Point(0, 0));
-        var bottomRight = this.viewport.imageToWindowCoordinates(this.getContentSize());
+        var topLeft = this.imageToWindowCoordinates(new $.Point(0, 0));
+        var bottomRight = this.imageToWindowCoordinates(this.getContentSize());
         return new $.Point(bottomRight.x - topLeft.x, bottomRight.y - topLeft.y);
     },