Optgroup tags inside another optgroup tags is not working

[adnan4077]

Hi, I am adnan. I need help. i am using selection tags with OptGroups. My optgroup tag is inside another optgroup tag to make it multilevel but it is showing only one optgroup label. My code is mentioned as follows.

<select>
  <optgroup label="Swedish Cars">
  	<optgroup label="Cars">
      <option value="volvo">Volvo</option>
      <option value="saab">Saab</option>
  	</optgroup>
    <optgroup label="Cars">
      <option value="volvo">Volvo</option>
      <option value="saab">Saab</option>
  	</optgroup>
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
  </optgroup>
  <optgroup label="German Cars">
    <option value="mercedes">Mercedes</option>
    <option value="audi">Audi</option>
  </optgroup>
</select>

The output i want to get get is like

But output i am getting in the theme is

in which "Swedish Cars" is missing at the top So plz guide me how to use multilevel optgroup in selection tags.