多选下拉placeholder问题

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    <mu-form-item label="多选下拉：">
            <mu-select v-model="form.select22" full-width multiple popover-class="formSelect2" placeholder="多选下拉">
                <mu-option v-for="option,index in optionData" :key="option" :label="option" :value="option"></mu-option>
            </mu-select>
        </mu-form-item>

多选的v-model如果是数组的话，placeholder不能用