diff --git a/lib/OpenLayers/Control/PanZoomBar.js b/lib/OpenLayers/Control/PanZoomBar.js
index 64f684915f..414e5b59b3 100644
--- a/lib/OpenLayers/Control/PanZoomBar.js
+++ b/lib/OpenLayers/Control/PanZoomBar.js
@@ -110,6 +110,15 @@ OpenLayers.Control.PanZoomBar.prototype =
             this.zoomStopHeight*(this.map.getZoomLevels()+1));
         return centered; 
     },
+    /* 
+     * @param evt
+     * This function is used to pass events that happen on the div, or the map,
+     * through to the slider, which then does its moving thing.
+     */
+    passEventToSlider:function(evt) {
+        this.sliderEvents.handleBrowserEvent(evt);
+    },
+    
     /*
      * divClick: Picks up on clicks directly on the zoombar div
      *           and sets the zoom level appropriately.
@@ -121,6 +130,11 @@ OpenLayers.Control.PanZoomBar.prototype =
         this.map.zoomTo(this.map.getZoomLevels() - levels);
         Event.stop(evt);
     },
+    
+    /* 
+     * @param evt
+     * event listener for clicks on the slider
+     */
     zoomBarDown:function(evt) {
         this.map.events.register("mousemove", this, this.passEventToSlider);
         this.map.events.register("mouseup", this, this.passEventToSlider);
@@ -129,9 +143,13 @@ OpenLayers.Control.PanZoomBar.prototype =
         this.div.style.cursor = "move";
         Event.stop(evt);
     },
-    passEventToSlider:function(evt) {
-        this.sliderEvents.handleBrowserEvent(evt);
-    },
+    
+    /*
+     * @param evt
+     * This is what happens when a click has occurred, and the client is dragging.
+     * Here we must ensure that the slider doesn't go beyond the bottom/top of the 
+     * zoombar div, as well as moving the slider to its new visual location
+     */
     zoomBarDrag:function(evt) {
         if (this.mouseDragStart != null) {
             var deltaY = this.mouseDragStart.y - evt.xy.y
@@ -145,16 +163,28 @@ OpenLayers.Control.PanZoomBar.prototype =
         }
         Event.stop(evt);
     },
+    
+    /* 
+     * @param evt
+     * Perform cleanup when a mouseup event is received -- discover new zoom level
+     * and switch to it.
+     */
     zoomBarUp:function(evt) {
-        this.div.style.cursor="default";
-        this.map.events.remove("mousemove");
-        this.map.events.remove("mouseup");
-        var deltaY = this.zoomStart.y - evt.xy.y
-        this.map.zoomTo(this.map.zoom + Math.round(deltaY/this.zoomStopHeight));
-        this.moveZoomBar();
-        this.mouseDragStart = null;
-        Event.stop(evt);
+        if (this.zoomStart) {
+            this.div.style.cursor="default";
+            this.map.events.remove("mousemove");
+            this.map.events.remove("mouseup");
+            var deltaY = this.zoomStart.y - evt.xy.y
+            this.map.zoomTo(this.map.zoom + Math.round(deltaY/this.zoomStopHeight));
+            this.moveZoomBar();
+            this.mouseDragStart = null;
+            Event.stop(evt);
+        }
     },
+    
+    /* 
+    * Change the location of the slider to match the current zoom level.
+    */
     moveZoomBar:function() {
         var newTop = 
             (this.map.getZoomLevels() - this.map.getZoom()) * this.zoomStopHeight