Browser send optgroup label

I have generated with form.fields_for and
the following html (this is just sample):
<select name="car[name]">
  <optgroup label="Swedish Cars">
    <option value="volvo">Volvo</option>
    <option value="saab">Saab</option>
  <optgroup label="German Cars">
    <option value="mercedes">Mercedes</option>
    <option value="audi">Audi</option>

when user submit form, in log i have params[:car][:name] = "Swedish
i did not success to reproduce this issue .. except replacing one of
"option"'s value with "Swedish Cars"
But optgroup in normal situation should not be selected ... maybe IE9
bug ? (i dont know what browser was that)