diff --git a/src/buttongroup.js b/src/buttongroup.js
index 9947641b..3898da56 100644
--- a/src/buttongroup.js
+++ b/src/buttongroup.js
@@ -112,6 +112,17 @@ $.ButtonGroup = function( options ) {
 /** @lends OpenSeadragon.ButtonGroup.prototype */
 $.ButtonGroup.prototype = {
 
+    /**
+     * Adds the given button to this button group.
+     *
+     * @functions
+     * @param {OpenSeadragon.Button} button
+     */
+    addButton: function( button ){
+        this.buttons.push(button);
+        this.element.appendChild(button.element);
+    },
+
     /**
      * TODO: Figure out why this is used on the public API and if a more useful
      * api can be created.
diff --git a/src/viewer.js b/src/viewer.js
index 82db55ec..af5bcf05 100644
--- a/src/viewer.js
+++ b/src/viewer.js
@@ -978,15 +978,14 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
         this.forceRedraw();
     },
 
-     /**
-     * Add custom button  for this viewer.
+    /**
+     * Adds the given button to this viewer.
      *
      * @functions
-     * @param {Element} CustomButton A reference to an element.
+     * @param {OpenSeadragon.Button} button
      */
-     addButton: function( Custombutton ){
-        this.buttonGroup.buttons.push(Custombutton);
-        this.buttonGroup.element.appendChild(Custombutton.element);
+    addButton: function( button ){
+        this.buttonGroup.addButton(button);
     },
 
     /**