diff --git a/caps/src/nsScriptSecurityManager.cpp b/caps/src/nsScriptSecurityManager.cpp
index 54a2535ed81..bd41bb8ea15 100644
--- a/caps/src/nsScriptSecurityManager.cpp
+++ b/caps/src/nsScriptSecurityManager.cpp
@@ -2442,9 +2442,9 @@ nsScriptSecurityManager::doGetObjectPrincipal(JSObject *aObj
             if (!(~jsClass->flags & (JSCLASS_HAS_PRIVATE |
                                      JSCLASS_PRIVATE_IS_NSISUPPORTS))) {
                 priv = (nsISupports *) js::GetObjectPrivate(aObj);
-            } else if ((jsClass->flags & JSCLASS_IS_DOMJSCLASS) &&
+            } else if (IsDOMClass(jsClass) &&
                        DOMJSClass::FromJSClass(jsClass)->mDOMObjectIsISupports) {
-                priv = UnwrapDOMObject<nsISupports>(aObj, jsClass);
+                priv = UnwrapDOMObject<nsISupports>(aObj);
             } else {
                 priv = nsnull;
             }
diff --git a/dom/bindings/BindingUtils.cpp b/dom/bindings/BindingUtils.cpp
index b124a13da46..be6d8fa4f76 100644
--- a/dom/bindings/BindingUtils.cpp
+++ b/dom/bindings/BindingUtils.cpp
@@ -340,11 +340,12 @@ QueryInterface(JSContext* cx, unsigned argc, JS::Value* vp)
 
   JSObject* obj = JSVAL_TO_OBJECT(thisv);
   JSClass* clasp = js::GetObjectJSClass(obj);
-  if (!IsDOMClass(clasp)) {
+  if (!IsDOMClass(clasp) ||
+      !DOMJSClass::FromJSClass(clasp)->mDOMObjectIsISupports) {
     return Throw<true>(cx, NS_ERROR_FAILURE);
   }
 
-  nsISupports* native = UnwrapDOMObject<nsISupports>(obj, clasp);
+  nsISupports* native = UnwrapDOMObject<nsISupports>(obj);
 
   if (argc < 1) {
     return Throw<true>(cx, NS_ERROR_XPC_NOT_ENOUGH_ARGS);
diff --git a/dom/bindings/BindingUtils.h b/dom/bindings/BindingUtils.h
index b67110bec54..7a36e5fa096 100644
--- a/dom/bindings/BindingUtils.h
+++ b/dom/bindings/BindingUtils.h
@@ -60,9 +60,15 @@ IsDOMClass(const JSClass* clasp)
   return clasp->flags & JSCLASS_IS_DOMJSCLASS;
 }
 
+inline bool
+IsDOMClass(const js::Class* clasp)
+{
+  return clasp->flags & JSCLASS_IS_DOMJSCLASS;
+}
+
 template <class T>
 inline T*
-UnwrapDOMObject(JSObject* obj, const JSClass* clasp)
+UnwrapDOMObject(JSObject* obj)
 {
   JS::Value val = js::GetReservedSlot(obj, DOM_OBJECT_SLOT);
   // XXXbz/khuey worker code tries to unwrap interface objects (which have
@@ -76,13 +82,6 @@ UnwrapDOMObject(JSObject* obj, const JSClass* clasp)
   return static_cast<T*>(val.toPrivate());
 }
 
-template <class T>
-inline T*
-UnwrapDOMObject(JSObject* obj, const js::Class* clasp)
-{
-  return UnwrapDOMObject<T>(obj, Jsvalify(clasp));
-}
-
 // Some callers don't want to set an exception when unwrappin fails
 // (for example, overload resolution uses unwrapping to tell what sort
 // of thing it's looking at).
@@ -120,7 +119,7 @@ UnwrapObject(JSContext* cx, JSObject* obj, U& value)
   DOMJSClass* domClass = DOMJSClass::FromJSClass(clasp);
   if (domClass->mInterfaceChain[PrototypeTraits<PrototypeID>::Depth] ==
       PrototypeID) {
-    value = UnwrapDOMObject<T>(obj, clasp);
+    value = UnwrapDOMObject<T>(obj);
     return NS_OK;
   }
 
diff --git a/dom/bindings/Codegen.py b/dom/bindings/Codegen.py
index e32050ee23c..6a898583bc3 100644
--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -464,8 +464,7 @@ class CGAbstractClassHook(CGAbstractStaticMethod):
 
     def definition_body_prologue(self):
         return """
-  MOZ_ASSERT(js::GetObjectJSClass(obj) == Class.ToJSClass());
-  %s* self = UnwrapDOMObject<%s>(obj, Class.ToJSClass());
+  %s* self = UnwrapDOMObject<%s>(obj);
 """ % (self.descriptor.nativeType, self.descriptor.nativeType)
 
     def definition_body(self):
diff --git a/dom/workers/Worker.cpp b/dom/workers/Worker.cpp
index ab1eb20d473..25ea433c158 100644
--- a/dom/workers/Worker.cpp
+++ b/dom/workers/Worker.cpp
@@ -217,7 +217,7 @@ private:
   Finalize(JSFreeOp* aFop, JSObject* aObj)
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
-    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj, Class());
+    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj);
     if (worker) {
       worker->_finalize(aFop);
     }
@@ -227,7 +227,7 @@ private:
   Trace(JSTracer* aTrc, JSObject* aObj)
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
-    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj, Class());
+    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj);
     if (worker) {
       worker->_trace(aTrc);
     }
@@ -358,7 +358,7 @@ private:
     if (aObj) {
       JSClass* classPtr = JS_GetClass(aObj);
       if (classPtr == Class()) {
-        return UnwrapDOMObject<WorkerPrivate>(aObj, Class());
+        return UnwrapDOMObject<WorkerPrivate>(aObj);
       }
     }
 
@@ -375,7 +375,7 @@ private:
   Finalize(JSFreeOp* aFop, JSObject* aObj)
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
-    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj, Class());
+    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj);
     if (worker) {
       worker->_finalize(aFop);
     }
@@ -385,7 +385,7 @@ private:
   Trace(JSTracer* aTrc, JSObject* aObj)
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
-    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj, Class());
+    WorkerPrivate* worker = UnwrapDOMObject<WorkerPrivate>(aObj);
     if (worker) {
       worker->_trace(aTrc);
     }
@@ -414,7 +414,7 @@ Worker::GetInstancePrivate(JSContext* aCx, JSObject* aObj,
 {
   JSClass* classPtr = JS_GetClass(aObj);
   if (classPtr == Class() || classPtr == ChromeWorker::Class()) {
-    return UnwrapDOMObject<WorkerPrivate>(aObj, classPtr);
+    return UnwrapDOMObject<WorkerPrivate>(aObj);
   }
 
   JS_ReportErrorNumber(aCx, js_GetErrorMessage, NULL, JSMSG_INCOMPATIBLE_PROTO,
diff --git a/dom/workers/WorkerScope.cpp b/dom/workers/WorkerScope.cpp
index d0b0051eee5..d9387f1375e 100644
--- a/dom/workers/WorkerScope.cpp
+++ b/dom/workers/WorkerScope.cpp
@@ -768,7 +768,7 @@ private:
   {
     JSClass* classPtr = JS_GetClass(aObj);
     if (classPtr == Class()) {
-      return UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj, classPtr);
+      return UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj);
     }
 
     JS_ReportErrorNumber(aCx, js_GetErrorMessage, NULL,
@@ -803,7 +803,7 @@ private:
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
     DedicatedWorkerGlobalScope* scope =
-      UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj, Class());
+      UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj);
     if (scope) {
       DestroyProtoOrIfaceCache(aObj);
       scope->_finalize(aFop);
@@ -815,7 +815,7 @@ private:
   {
     JS_ASSERT(JS_GetClass(aObj) == Class());
     DedicatedWorkerGlobalScope* scope =
-      UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj, Class());
+      UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj);
     if (scope) {
       mozilla::dom::TraceProtoOrIfaceCache(aTrc, aObj);
       scope->_trace(aTrc);
@@ -888,7 +888,7 @@ WorkerGlobalScope::GetInstancePrivate(JSContext* aCx, JSObject* aObj,
   JS_ASSERT(classPtr != Class());
 
   if (classPtr == DedicatedWorkerGlobalScope::Class()) {
-    return UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj, classPtr);
+    return UnwrapDOMObject<DedicatedWorkerGlobalScope>(aObj);
   }
 
   JS_ReportErrorNumber(aCx, js_GetErrorMessage, NULL, JSMSG_INCOMPATIBLE_PROTO,
diff --git a/js/xpconnect/src/XPCConvert.cpp b/js/xpconnect/src/XPCConvert.cpp
index 9df149b2b94..8e730241c92 100644
--- a/js/xpconnect/src/XPCConvert.cpp
+++ b/js/xpconnect/src/XPCConvert.cpp
@@ -74,9 +74,9 @@ XPCConvert::GetISupportsFromJSObject(JSObject* obj, nsISupports** iface)
         *iface = (nsISupports*) xpc_GetJSPrivate(obj);
         return true;
     }
-    if (jsclass && (jsclass->flags & JSCLASS_IS_DOMJSCLASS) &&
+    if (jsclass && IsDOMClass(jsclass) &&
         DOMJSClass::FromJSClass(jsclass)->mDOMObjectIsISupports) {
-        *iface = UnwrapDOMObject<nsISupports>(obj, jsclass);
+        *iface = UnwrapDOMObject<nsISupports>(obj);
         return true;
     }
     return false;
diff --git a/js/xpconnect/src/XPCJSID.cpp b/js/xpconnect/src/XPCJSID.cpp
index 06ca95107b6..14a104463d5 100644
--- a/js/xpconnect/src/XPCJSID.cpp
+++ b/js/xpconnect/src/XPCJSID.cpp
@@ -489,8 +489,7 @@ nsJSIID::HasInstance(nsIXPConnectWrappedNative *wrapper,
                               js::GetObjectJSClass(obj))->mDOMObjectIsISupports,
                          "This only works on nsISupports classes!");
             identity =
-                mozilla::dom::UnwrapDOMObject<nsISupports>(obj,
-                                                           js::GetObjectJSClass(obj));
+                mozilla::dom::UnwrapDOMObject<nsISupports>(obj);
         } else {
             identity = nsnull;
         }
diff --git a/js/xpconnect/src/XPCJSRuntime.cpp b/js/xpconnect/src/XPCJSRuntime.cpp
index b75d0295877..8a562231bd2 100644
--- a/js/xpconnect/src/XPCJSRuntime.cpp
+++ b/js/xpconnect/src/XPCJSRuntime.cpp
@@ -25,6 +25,7 @@
 #include "jsfriendapi.h"
 #include "js/MemoryMetrics.h"
 #include "mozilla/dom/DOMJSClass.h"
+#include "mozilla/dom/BindingUtils.h"
 
 #include "nsJSPrincipals.h"
 
@@ -460,14 +461,14 @@ SuspectDOMExpandos(nsPtrHashKey<JSObject> *key, void *arg)
     JSObject* obj = key->GetKey();
     nsISupports* native = nsnull;
     if (js::IsProxy(obj)) {
-        NS_ASSERTION(mozilla::dom::binding::instanceIsProxy(obj),
+        NS_ASSERTION(dom::binding::instanceIsProxy(obj),
                      "Not a DOM proxy?");
         native = static_cast<nsISupports*>(js::GetProxyPrivate(obj).toPrivate());
     }
     else {
-        NS_ASSERTION(mozilla::dom::DOMJSClass::FromJSClass(JS_GetClass(obj))->mDOMObjectIsISupports,
+        NS_ASSERTION(dom::DOMJSClass::FromJSClass(JS_GetClass(obj))->mDOMObjectIsISupports,
                      "Someone added a wrapper for a non-nsISupports native to DOMExpandos!");
-        native = static_cast<nsISupports*>(js::GetReservedSlot(obj, DOM_OBJECT_SLOT).toPrivate());
+        native = dom::UnwrapDOMObject<nsISupports>(obj);
     }
     closure->cb->NoteXPCOMRoot(native);
     return PL_DHASH_NEXT;
diff --git a/js/xpconnect/src/XPCWrappedNativeScope.cpp b/js/xpconnect/src/XPCWrappedNativeScope.cpp
index 1f62ddd42c6..680a3c5b294 100644
--- a/js/xpconnect/src/XPCWrappedNativeScope.cpp
+++ b/js/xpconnect/src/XPCWrappedNativeScope.cpp
@@ -223,9 +223,9 @@ XPCWrappedNativeScope::SetGlobal(XPCCallContext& ccx, JSObject* aGlobal,
             // Our global has an nsISupports native pointer.  Let's
             // see whether it's what we want.
             priv = static_cast<nsISupports*>(xpc_GetJSPrivate(aGlobal));
-        } else if ((jsClass->flags & JSCLASS_IS_DOMJSCLASS) &&
+        } else if (dom::IsDOMClass(jsClass) &&
                    dom::DOMJSClass::FromJSClass(jsClass)->mDOMObjectIsISupports) {
-            priv = dom::UnwrapDOMObject<nsISupports>(aGlobal, jsClass);
+            priv = dom::UnwrapDOMObject<nsISupports>(aGlobal);
         } else {
             priv = nsnull;
         }
diff --git a/js/xpconnect/src/nsXPConnect.cpp b/js/xpconnect/src/nsXPConnect.cpp
index f1bc49f5c66..afe264c5c77 100644
--- a/js/xpconnect/src/nsXPConnect.cpp
+++ b/js/xpconnect/src/nsXPConnect.cpp
@@ -870,10 +870,10 @@ nsXPConnect::Traverse(void *p, nsCycleCollectionTraversalCallback &cb)
         nsISupports *identity =
             static_cast<nsISupports*>(js::GetProxyPrivate(obj).toPrivate());
         cb.NoteXPCOMChild(identity);
-    } else if ((clazz->flags & JSCLASS_IS_DOMJSCLASS) &&
+    } else if (IsDOMClass(clazz) &&
                DOMJSClass::FromJSClass(clazz)->mDOMObjectIsISupports) {
         NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(cb, "UnwrapDOMObject(obj)");
-        nsISupports *identity = UnwrapDOMObject<nsISupports>(obj, clazz);
+        nsISupports *identity = UnwrapDOMObject<nsISupports>(obj);
         cb.NoteXPCOMChild(identity);
     }