微信小程序 picker 中range-key的坑

<picker class='fr' bindchange="onChangeBuild" range-key="{{'num'}}" value="{{buildIndex}}" range="{{aBuilding}}">
        <view class="picker">
             罗兰{{aBuilding[buildIndex].num}}幢
        </view>
</picker>

range-key中的值要么 range-key="{{'num'}}"，要么range-key="num'"