--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -3419,37 +3419,37 @@ IsStandardPrototype(JSObject *obj, JSPro
     return v.isObject() && obj == &v.toObject();
 }
 
 JSProtoKey
 JS::IdentifyStandardInstance(JSObject *obj)
 {
     // Note: The prototype shares its JSClass with instances.
     JS_ASSERT(!obj->is<CrossCompartmentWrapperObject>());
-    JSProtoKey key = JSCLASS_CACHED_PROTO_KEY(obj->getClass());
+    JSProtoKey key = StandardProtoKeyOrNull(obj);
     if (key != JSProto_Null && !IsStandardPrototype(obj, key))
         return key;
     return JSProto_Null;
 }
 
 JSProtoKey
 JS::IdentifyStandardPrototype(JSObject *obj)
 {
     // Note: The prototype shares its JSClass with instances.
     JS_ASSERT(!obj->is<CrossCompartmentWrapperObject>());
-    JSProtoKey key = JSCLASS_CACHED_PROTO_KEY(obj->getClass());
+    JSProtoKey key = StandardProtoKeyOrNull(obj);
     if (key != JSProto_Null && IsStandardPrototype(obj, key))
         return key;
     return JSProto_Null;
 }
 
 JSProtoKey
 JS::IdentifyStandardInstanceOrPrototype(JSObject *obj)
 {
-    return JSCLASS_CACHED_PROTO_KEY(obj->getClass());
+    return StandardProtoKeyOrNull(obj);
 }
 
 JSProtoKey
 JS::IdentifyStandardConstructor(JSObject *obj)
 {
     // Note that NATIVE_CTOR does not imply that we are a standard constructor,
     // but the converse is true (at least until we start having self-hosted
     // constructors for standard classes). This lets us avoid a costly loop for

--- a/js/src/vm/GlobalObject.h
+++ b/js/src/vm/GlobalObject.h
@@ -802,16 +802,25 @@ GenericCreatePrototype(JSContext *cx, JS
     MOZ_ASSERT(key != JSProto_Object);
     JSProtoKey parentKey = ParentKeyForStandardClass(key);
     if (!GlobalObject::ensureConstructor(cx, cx->global(), parentKey))
         return nullptr;
     JSObject *parentProto = &cx->global()->getPrototype(parentKey).toObject();
     return cx->global()->createBlankPrototypeInheriting(cx, clasp, *parentProto);
 }
 
+inline JSProtoKey
+StandardProtoKeyOrNull(const JSObject *obj)
+{
+    JSProtoKey key = JSCLASS_CACHED_PROTO_KEY(obj->getClass());
+    if (key == JSProto_Error)
+        return GetExceptionProtoKey(obj->as<ErrorObject>().type());
+    return key;
+}
+
 } // namespace js
 
 template<>
 inline bool
 JSObject::is<js::GlobalObject>() const
 {
     return !!(getClass()->flags & JSCLASS_IS_GLOBAL);
 }