vue-ant-design中的select組件中value和placeholder同時設置時placeholder無效

<a-select  placeholder="請選擇性別"  :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中定義：

sex:undefined,