Problemer med javascript indhold i form

jeg har lavet følgende men det virker ikke :-/ den siger Mainlang er undefined hvis jeg sætter min <form> uden om, hvad kan man gøre anderledes ?

<form>
<script language="javascript">

function Dropper()
{
    oSelect = document.getElementById('Mainlang');
    FNr = Mainlang[Mainlang.selectedIndex].value;

    if (FNr == "0")
    {
        document.getElementById('Drop0').style.display="block";
        document.getElementById('Drop1').style.display="none";
        document.getElementById('Drop2').style.display="none";
        document.getElementById('Drop3').style.display="none";
        document.getElementById('DropS').style.display="none";
    }
    else if (FNr == "1")
    {
        document.getElementById('Drop0').style.display="none";
        document.getElementById('Drop1').style.display="block";
        document.getElementById('Drop2').style.display="none";
        document.getElementById('Drop3').style.display="none";
        document.getElementById('DropS').style.display="none";
    }
    else if (FNr == "2")
    {
        document.getElementById('Drop0').style.display="none";
        document.getElementById('Drop1').style.display="none";
        document.getElementById('Drop2').style.display="block";
        document.getElementById('Drop3').style.display="none";
        document.getElementById('DropS').style.display="none";
    }
    else if (FNr == "3")
    {
        document.getElementById('Drop0').style.display="none";
        document.getElementById('Drop1').style.display="none";
        document.getElementById('Drop2').style.display="none";
        document.getElementById('Drop3').style.display="block";
        document.getElementById('DropS').style.display="none";
    }

    else if (FNr == "6")
    {
        document.getElementById('Drop0').style.display="none";
        document.getElementById('Drop1').style.display="none";
        document.getElementById('Drop2').style.display="none";
        document.getElementById('Drop3').style.display="none";
        document.getElementById('DropS').style.display="block";
    }

}

</script>

<select name="Mainlang" onChange="Dropper();">
<option selected value="6">Choose Module</option>
<option value="0">t0</option>
<option value="1">t1</option>
<option value="2">t2</option>
<option value="3">t3</option>

</select>
   


<select size="1" name="DropS" style="display:'block'" disabled="true">
<option></option>
</select>

<select size="1" name="Drop0" style="display:'none'">
<option>3534</option>
</select>

<select size="1" name="Drop1" style="display:'none'">
<option>tytyyt</option>
</select>

<select size="1" name="Drop2" style="display:'none'">
<option>455445</option>
</select>

<select size="1" name="Drop3" style="display:'none'">
<option>544545</option>
</select>
</form>