diff --git a/CHANGELOG b/CHANGELOG
index 95fb238..b2bfd82 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* Make sure `_extendedByPrototype`, `_countedByPrototype`, and `prototypeUID` node expandos are accessed with `typeof` to prevent errors in some environments. [#354 state:resolved] (Hilberty, kangax, Andrew Dupont)
+
 * Fix issue where Opera 9.x returns incorrect results on certain Selector queries with descendant combinators. [#395 state:resolved] (Arpan, fearphage, kangax, Andrew Dupont)
 
 * Null out references to elements in cache on page unload. Need this in addition to the Event#stopObserving calls to clean up memory leaks. [#425 state:resolved] (ykphuah, mr_justin, Andrew Dupont)
diff --git a/src/dom/dom.js b/src/dom/dom.js
index 129d794..0b15981 100644
--- a/src/dom/dom.js
+++ b/src/dom/dom.js
@@ -1508,7 +1508,9 @@ Element.extend = (function() {
   var Methods = { }, ByTag = Element.Methods.ByTag;
   
   var extend = Object.extend(function(element) {
-    if (!element || element._extendedByPrototype || 
+    // need to use actual `typeof` operator 
+    // to prevent errors in some environments (when accessing node expandos)
+    if (!element || typeof element._extendedByPrototype != 'undefined' || 
         element.nodeType != 1 || element == window) return element;
 
     var methods = Object.clone(Methods),
@@ -1735,7 +1737,7 @@ Element.addMethods({
     if (element === window) {
       uid = 0;
     } else {
-      if (Object.isUndefined(element._prototypeUID))
+      if (typeof element._prototypeUID === "undefined")
         element._prototypeUID = [Element.Storage.UID++];
       uid = element._prototypeUID[0];
     }
diff --git a/src/dom/selector.js b/src/dom/selector.js
index 0e3e486..b37e539 100644
--- a/src/dom/selector.js
+++ b/src/dom/selector.js
@@ -439,7 +439,8 @@ Object.extend(Selector, {
       if (nodes.length == 0) return nodes;
       var results = [], n;
       for (var i = 0, l = nodes.length; i < l; i++)
-        if (!(n = nodes[i])._countedByPrototype) {
+        // use `typeof` operator to prevent errors
+        if (typeof (n = nodes[i])._countedByPrototype == 'undefined') {
           n._countedByPrototype = Prototype.emptyFunction;
           results.push(Element.extend(n));
         }