diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index 825d27790368..65d5c8f89456 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -3424,7 +3424,7 @@ 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;
@@ -3435,7 +3435,7 @@ 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;
@@ -3444,7 +3444,7 @@ JS::IdentifyStandardPrototype(JSObject *obj)
 JSProtoKey
 JS::IdentifyStandardInstanceOrPrototype(JSObject *obj)
 {
-    return JSCLASS_CACHED_PROTO_KEY(obj->getClass());
+    return StandardProtoKeyOrNull(obj);
 }
 
 JSProtoKey
diff --git a/js/src/vm/GlobalObject.h b/js/src/vm/GlobalObject.h
index bd0a92a0826d..27e61ff654a7 100644
--- a/js/src/vm/GlobalObject.h
+++ b/js/src/vm/GlobalObject.h
@@ -807,6 +807,15 @@ GenericCreatePrototype(JSContext *cx, JSProtoKey key)
     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<>