diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 0f475b58b74..53197137646 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -1529,6 +1529,7 @@ NewPropertyDescriptorObject(JSContext *cx, const PropertyDescriptor *desc, Value
 void
 PropDesc::initFromPropertyDescriptor(const PropertyDescriptor &desc)
 {
+    isUndefined_ = false;
     pd_.setUndefined();
     attrs = uint8_t(desc.attrs);
     JS_ASSERT_IF(attrs & JSPROP_READONLY, !(attrs & (JSPROP_GETTER | JSPROP_SETTER)));
@@ -1560,6 +1561,8 @@ PropDesc::initFromPropertyDescriptor(const PropertyDescriptor &desc)
 bool
 PropDesc::makeObject(JSContext *cx)
 {
+    MOZ_ASSERT(!isUndefined());
+
     JSObject *obj = NewBuiltinClassInstance(cx, &ObjectClass);
     if (!obj)
         return false;
@@ -1728,21 +1731,6 @@ HasProperty(JSContext *cx, JSObject *obj, jsid id, Value *vp, bool *foundp)
     return !!obj->getGeneric(cx, id, vp);
 }
 
-PropDesc::PropDesc()
-  : pd_(UndefinedValue()),
-    value_(UndefinedValue()),
-    get_(UndefinedValue()),
-    set_(UndefinedValue()),
-    attrs(0),
-    hasGet_(false),
-    hasSet_(false),
-    hasValue_(false),
-    hasWritable_(false),
-    hasEnumerable_(false),
-    hasConfigurable_(false)
-{
-}
-
 bool
 PropDesc::initialize(JSContext *cx, const Value &origval, bool checkAccessors)
 {
@@ -1758,7 +1746,12 @@ PropDesc::initialize(JSContext *cx, const Value &origval, bool checkAccessors)
     /* Make a copy of the descriptor. We might need it later. */
     pd_ = v;
 
-    /* Start with the proper defaults. */
+    isUndefined_ = false;
+
+    /*
+     * Start with the proper defaults.  XXX shouldn't be necessary when we get
+     * rid of PropDesc::attributes()
+     */
     attrs = JSPROP_PERMANENT | JSPROP_READONLY;
 
     bool found;
diff --git a/js/src/vm/ObjectImpl.cpp b/js/src/vm/ObjectImpl.cpp
index caf22adaac1..64dd1952dc6 100644
--- a/js/src/vm/ObjectImpl.cpp
+++ b/js/src/vm/ObjectImpl.cpp
@@ -20,12 +20,27 @@
 
 using namespace js;
 
+PropDesc::PropDesc()
+  : pd_(UndefinedValue()),
+    value_(UndefinedValue()),
+    get_(UndefinedValue()),
+    set_(UndefinedValue()),
+    attrs(0),
+    hasGet_(false),
+    hasSet_(false),
+    hasValue_(false),
+    hasWritable_(false),
+    hasEnumerable_(false),
+    hasConfigurable_(false),
+    isUndefined_(true)
+{
+}
+
 bool
 PropDesc::checkGetter(JSContext *cx)
 {
-    if (hasGet()) {
-        const Value &get = getterValue();
-        if (!js_IsCallable(get) && !get.isUndefined()) {
+    if (hasGet_) {
+        if (!js_IsCallable(get_) && !get_.isUndefined()) {
             JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_BAD_GET_SET_FIELD,
                                  js_getter_str);
             return false;
@@ -37,9 +52,8 @@ PropDesc::checkGetter(JSContext *cx)
 bool
 PropDesc::checkSetter(JSContext *cx)
 {
-    if (hasSet()) {
-        const Value &set = setterValue();
-        if (!js_IsCallable(set) && !set.isUndefined()) {
+    if (hasSet_) {
+        if (!js_IsCallable(set_) && !set_.isUndefined()) {
             JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_BAD_GET_SET_FIELD,
                                  js_setter_str);
             return false;
@@ -68,6 +82,8 @@ bool
 PropDesc::unwrapDebuggerObjectsInto(JSContext *cx, Debugger *dbg, JSObject *obj,
                                     PropDesc *unwrapped) const
 {
+    MOZ_ASSERT(!isUndefined());
+
     *unwrapped = *this;
 
     if (unwrapped->hasValue()) {
@@ -106,6 +122,8 @@ bool
 PropDesc::wrapInto(JSContext *cx, JSObject *obj, const jsid &id, jsid *wrappedId,
                    PropDesc *desc) const
 {
+    MOZ_ASSERT(!isUndefined());
+
     JSCompartment *comp = cx->compartment;
 
     *wrappedId = id;
diff --git a/js/src/vm/ObjectImpl.h b/js/src/vm/ObjectImpl.h
index 538ebe254f7..dfabb4e11fe 100644
--- a/js/src/vm/ObjectImpl.h
+++ b/js/src/vm/ObjectImpl.h
@@ -61,6 +61,9 @@ struct PropDesc {
     bool hasEnumerable_ : 1;
     bool hasConfigurable_ : 1;
 
+    /* Or maybe this represents a property's absence, and it's undefined. */
+    bool isUndefined_ : 1;
+
   public:
     friend class AutoPropDescArrayRooter;
     friend void JS::AutoGCRooter::trace(JSTracer *trc);
@@ -91,68 +94,79 @@ struct PropDesc {
     void initFromPropertyDescriptor(const PropertyDescriptor &desc);
     bool makeObject(JSContext *cx);
 
-    bool hasGet() const { return hasGet_; }
-    bool hasSet() const { return hasSet_; }
-    bool hasValue() const { return hasValue_; }
-    bool hasWritable() const { return hasWritable_; }
-    bool hasEnumerable() const { return hasEnumerable_; }
-    bool hasConfigurable() const { return hasConfigurable_; }
+    void setUndefined() { isUndefined_ = true; }
 
-    Value pd() const { return pd_; }
+    bool isUndefined() const { return isUndefined_; }
+
+    bool hasGet() const { MOZ_ASSERT(!isUndefined()); return hasGet_; }
+    bool hasSet() const { MOZ_ASSERT(!isUndefined()); return hasSet_; }
+    bool hasValue() const { MOZ_ASSERT(!isUndefined()); return hasValue_; }
+    bool hasWritable() const { MOZ_ASSERT(!isUndefined()); return hasWritable_; }
+    bool hasEnumerable() const { MOZ_ASSERT(!isUndefined()); return hasEnumerable_; }
+    bool hasConfigurable() const { MOZ_ASSERT(!isUndefined()); return hasConfigurable_; }
+
+    Value pd() const { MOZ_ASSERT(!isUndefined()); return pd_; }
     void clearPd() { pd_ = UndefinedValue(); }
 
-    uint8_t attributes() const { return attrs; }
+    uint8_t attributes() const { MOZ_ASSERT(!isUndefined()); return attrs; }
 
     /* 8.10.1 IsAccessorDescriptor(desc) */
     bool isAccessorDescriptor() const {
-        return hasGet_ || hasSet_;
+        return !isUndefined() && (hasGet() || hasSet());
     }
 
     /* 8.10.2 IsDataDescriptor(desc) */
     bool isDataDescriptor() const {
-        return hasValue_ || hasWritable_;
+        return !isUndefined() && (hasValue() || hasWritable());
     }
 
     /* 8.10.3 IsGenericDescriptor(desc) */
     bool isGenericDescriptor() const {
-        return !isAccessorDescriptor() && !isDataDescriptor();
+        return !isUndefined() && !isAccessorDescriptor() && !isDataDescriptor();
     }
 
     bool configurable() const {
-        MOZ_ASSERT(hasConfigurable_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasConfigurable());
         return (attrs & JSPROP_PERMANENT) == 0;
     }
 
     bool enumerable() const {
-        MOZ_ASSERT(hasEnumerable_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasEnumerable());
         return (attrs & JSPROP_ENUMERATE) != 0;
     }
 
     bool writable() const {
-        MOZ_ASSERT(hasWritable_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasWritable());
         return (attrs & JSPROP_READONLY) == 0;
     }
 
     const Value & value() const {
-        MOZ_ASSERT(hasValue_);
+        MOZ_ASSERT(hasValue());
         return value_;
     }
 
     JSObject * getterObject() const {
-        MOZ_ASSERT(hasGet_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasGet());
         return get_.isUndefined() ? NULL : &get_.toObject();
     }
     JSObject * setterObject() const {
-        MOZ_ASSERT(hasSet_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasSet());
         return set_.isUndefined() ? NULL : &set_.toObject();
     }
 
     const Value & getterValue() const {
-        MOZ_ASSERT(hasGet_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasGet());
         return get_;
     }
     const Value & setterValue() const {
-        MOZ_ASSERT(hasSet_);
+        MOZ_ASSERT(!isUndefined());
+        MOZ_ASSERT(hasSet());
         return set_;
     }