diff --git a/src/viewport.js b/src/viewport.js
index 7d5418bd..d34ab539 100644
--- a/src/viewport.js
+++ b/src/viewport.js
@@ -1208,6 +1208,7 @@ $.Viewport.prototype = {
      * in image coordinate system.
      * @param {Number} [pixelWidth] the width in pixel of the rectangle.
      * @param {Number} [pixelHeight] the height in pixel of the rectangle.
+     * @returns {OpenSeadragon.Rect} This image's bounds in viewport coordinates
      */
     imageToViewportRectangle: function(imageX, imageY, pixelWidth, pixelHeight) {
         var rect = imageX;