Bootstrap导航栏搜索表单输入有未应用的错误类

<nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Logo</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">About</a>
      </li>
    </ul>
    <form class="form-inline my-2 my-lg-0">
      <div class="input-group">
        <input class="form-control" type="search" placeholder="Search" aria-label="Search">
        <div class="input-group-append">
          <button class="btn btn-outline-secondary" type="submit">Search</button>
        </div>
      </div>
    </form>
  </div>
</nav>