bootstrap btn-组高度不均匀

<div class="btn-group-vertical">
  <button type="button" class="btn btn-primary">按钮1</button>
  <button type="button" class="btn btn-primary">按钮2</button>
  <button type="button" class="btn btn-primary">按钮3</button>
</div>

<style>
  .btn-custom {
    height: 40px !important;
  }
</style>

<div class="btn-group">
  <button type="button" class="btn btn-primary btn-custom">按钮1</button>
  <button type="button" class="btn btn-primary btn-custom">按钮2</button>
  <button type="button" class="btn btn-primary btn-custom">按钮3</button>
</div>