vue屬性綁定里，怎么把num轉(zhuǎn)換成str？

<template>
    <div>  
        <div v-for="secMenu in menuData">
      <el-menu>
        <el-submenu :index="secMenu.id">
          <template slot="title">
            <i class="el-icon-location"></i>
            <span slot="title">{{secMenu.title}}</span>
          </template>
          <div v-for="item in secMenu.child">
            <el-menu-item :index='secMenu.id+"-"+item.id'>{{item.title}}</el-menu-item>
          </div>
        </el-submenu>
      </el-menu>
        </div>
    </div>
</template>

我這樣寫瀏覽器會報錯

[Vue warn]: Invalid prop: type check failed for prop "index". Expected String, got Number.

應該怎么寫才對？