Få Onclick funktionen til at virke på <select>

<script type="text/javascript">
function spiltypeChange(sel){
  var val = sel.value;
  var sp1 = document.getElementById('Spiltype1');
  var sp2 = document.getElementById('Spiltype2');
  sp1.style.display = (val == "Systemspil")?"":"none";
  sp2.style.display = (val == "Under/over")?"":"none";
}
</script>
<select name='Spiltype' onchange="spiltypeChange(this);">
<option value=''>Spiltype:</option>
<option value='Single'>Single</option>
<option value='Systemspil'>Systemspil</option>
<option value='Under/over'>Under/over</option>
</select>
<select id="Spiltype1" name='Spiltype1' style="display:none">
<option value=''>Spil1:</option>
<option value='2/3'>2/3</option>
<option value='2/4'>2/4</option>
<option value='2/5'>2/5</option>
<option value='3/4'>3/4</option>
<option value='3/5'>3/5</option>
<option value='3/6'>3/6</option>
<option value='4/5'>4/5</option>
<option value='4/6'>4/6</option>
<option value='5/6'>5/6</option>
<option value='5/7'>5/7</option>
</select>
<select id="Spiltype2" name='Spiltype2' style="display:none">
<option value=''>Spil2:</option>
<option value='0.5'>0.5</option>
<option value='1.5'>1.5</option>
<option value='2.5'>2.5</option>
<option value='3.5'>3.5</option>
<option value='4.5'>4.5</option>
<option value='5.5'>5.5</option>
<option value='6.5'>6.5</option>
<option value='7.5'>7.5</option>
<option value='8.5'>8.5</option>
</select>