vue-ant-design中的select组件中value和placeholder同时设置时placeholder无效

<a-select placeholder="请选择性别"  :value='sex' @change="handleChangeSelect">

                     <a-select-option value="">

                        未知

                    </a-select-option>

                    <a-select-option value="0">

                        男

                    </a-select-option>

                    <a-select-option value="1">

                        女

                    </a-select-option>

                </a-select>

需要在vue的data中定义：

sex:undefined,