public static RuntimeException notFunctionError(Object value,
                                                    Object messageHelper)
    {
        // Use value for better error reporting
        String msg = (messageHelper == null)
                     ? "null" : messageHelper.toString();
        if (value == Scriptable.NOT_FOUND) {
            return typeError1("msg.function.not.found", msg);
        }
        return typeError2("msg.isnt.function", msg, typeof(value));
    }