diff --git a/src/viewport.js b/src/viewport.js
index 5095a9d5..5a70de16 100644
--- a/src/viewport.js
+++ b/src/viewport.js
@@ -1134,7 +1134,7 @@ $.Viewport.prototype = {
         if (this.viewer) {
             var count = this.viewer.world.getItemCount();
             if (count > 1) {
-                $.console.error('[Viewport.imageToViewportCoordinates] is not accurate ' + 
+                $.console.error('[Viewport.imageToViewportCoordinates] is not accurate ' +
                     'with multi-image; use TiledImage.imageToViewportCoordinates instead.');
             } else if (count === 1) {
                 // It is better to use TiledImage.viewportToImageCoordinates
@@ -1276,10 +1276,12 @@ $.Viewport.prototype = {
      * @param {OpenSeadragon.Point} pixel
      * @returns {OpenSeadragon.Point}
      */
-    windowToImageCoordinates: function( pixel ) {
+    windowToImageCoordinates: function(pixel) {
+        $.console.assert(this.viewer,
+            "[Viewport.windowToImageCoordinates] the viewport must have a viewer.");
         var viewerCoordinates = pixel.minus(
-                OpenSeadragon.getElementPosition( this.viewer.element ));
-        return this.viewerElementToImageCoordinates( viewerCoordinates );
+                $.getElementPosition(this.viewer.element));
+        return this.viewerElementToImageCoordinates(viewerCoordinates);
     },
 
     /**
@@ -1288,10 +1290,12 @@ $.Viewport.prototype = {
      * @param {OpenSeadragon.Point} pixel
      * @returns {OpenSeadragon.Point}
      */
-    imageToWindowCoordinates: function( pixel ) {
-        var viewerCoordinates = this.imageToViewerElementCoordinates( pixel );
+    imageToWindowCoordinates: function(pixel) {
+        $.console.assert(this.viewer,
+            "[Viewport.imageToWindowCoordinates] the viewport must have a viewer.");
+        var viewerCoordinates = this.imageToViewerElementCoordinates(pixel);
         return viewerCoordinates.plus(
-                OpenSeadragon.getElementPosition( this.viewer.element ));
+                $.getElementPosition(this.viewer.element));
     },
 
     /**
@@ -1347,10 +1351,12 @@ $.Viewport.prototype = {
      * @param {OpenSeadragon.Point} pixel
      * @returns {OpenSeadragon.Point}
      */
-    windowToViewportCoordinates: function( pixel ) {
+    windowToViewportCoordinates: function(pixel) {
+        $.console.assert(this.viewer,
+            "[Viewport.windowToViewportCoordinates] the viewport must have a viewer.");
         var viewerCoordinates = pixel.minus(
-                OpenSeadragon.getElementPosition( this.viewer.element ));
-        return this.viewerElementToViewportCoordinates( viewerCoordinates );
+                $.getElementPosition(this.viewer.element));
+        return this.viewerElementToViewportCoordinates(viewerCoordinates);
     },
 
     /**
@@ -1358,10 +1364,12 @@ $.Viewport.prototype = {
      * @param {OpenSeadragon.Point} point
      * @returns {OpenSeadragon.Point}
      */
-    viewportToWindowCoordinates: function( point ) {
-        var viewerCoordinates = this.viewportToViewerElementCoordinates( point );
+    viewportToWindowCoordinates: function(point) {
+        $.console.assert(this.viewer,
+            "[Viewport.viewportToWindowCoordinates] the viewport must have a viewer.");
+        var viewerCoordinates = this.viewportToViewerElementCoordinates(point);
         return viewerCoordinates.plus(
-                OpenSeadragon.getElementPosition( this.viewer.element ));
+                $.getElementPosition(this.viewer.element));
     },
 
     /**
@@ -1380,7 +1388,7 @@ $.Viewport.prototype = {
         if (this.viewer) {
             var count = this.viewer.world.getItemCount();
             if (count > 1) {
-                $.console.error('[Viewport.viewportToImageZoom] is not ' + 
+                $.console.error('[Viewport.viewportToImageZoom] is not ' +
                     'accurate with multi-image.');
             } else if (count === 1) {
                 // It is better to use TiledImage.viewportToImageZoom