Dropdown onclick功能不起作用，如果我添加span内侧按钮来添加字体可怕的图标

<div class="dropdown">
  <button class="dropdown-toggle" onclick="toggleDropdown()">Dropdown</button>
  <div class="dropdown-menu">
    <span id="icon" class="fa fa-smile-o"></span>
    <a href="#">Option 1</a>
    <a href="#">Option 2</a>
    <a href="#">Option 3</a>
  </div>
</div>

.dropdown {
  position: relative;
  display: inline-block;
}

.dropdown-toggle {
  background-color: #f1f1f1;
  border: none;
  color: black;
  padding: 10px;
  cursor: pointer;
}

.dropdown-menu {
  display: none;
  position: absolute;
  background-color: #f9f9f9;
  min-width: 160px;
  box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
  z-index: 1;
}

.dropdown-menu a {
  color: black;
  padding: 12px 16px;
  text-decoration: none;
  display: block;
}

.dropdown-menu a:hover {
  background-color: #f1f1f1;
}

#icon {
  font-size: 20px;
  margin-right: 5px;
}

function toggleDropdown() {
  var dropdownMenu = document.getElementsByClassName("dropdown-menu")[0];
  if (dropdownMenu.style.display === "none") {
    dropdownMenu.style.display = "block";
  } else {
    dropdownMenu.style.display = "none";
  }
}