Bootstrap Modal无法滚动

<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <div class="modal-header">
        <h4 class="modal-title" id="myModalLabel">Modal标题</h4>
      </div>
      <div class="modal-body">
        <div class="modal-scrollable">
          <!-- 这里是Modal的内容 -->
        </div>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-default" data-dismiss="modal">关闭</button>
        <button type="button" class="btn btn-primary">保存</button>
      </div>
    </div>
  </div>
</div>

.modal-scrollable {
  height: 300px;
  overflow-y: auto;
}