el-cascader表单校验不生效

<el-form
    :model="form"
    :rules="rules"
     ref="form">
<el-form-item label="组织" prop="organizationList">
 <el-cascader v-model="form.organizationList"
                        :options="compOption"
                        :props="{ multiple: false,checkStrictly: true}"
                        collapse-tags
                        is-leaf
                        clearable/>
</el-form-item>
</el-form>

data(){
  const validateOrg = (rule, value, callback) => { 
      if (!this.form.organizationList.length) {
        callback(new Error('组织不能为空'))
      } else {
        callback()
      }
    }
  return {
    rules:{
     organizationList: [{ type: 'array', required: true, validator: validateOrg }],
    }
  }
}